build: the themes ship compiled, libsass is no longer installed

A theme change compiled Bootswatch with libsass (34 MB, upstream
deprecated) into two tracked files, which updates, rollbacks and new
Docker containers then reset. The 27 themes now ship as
static/css/themes/<theme>.min.css, built by "manage.py build_themes";
the Settings page only saves the theme name and every page links that
file (zephyr when no settings are in context, as on the 500 page).

libsass moves to dev/requirements.txt; a test checks that each shipped
file is its theme compiled from dev/scss. The "Theme SASS Path" setting
is removed, and the theme default "flaty", a typo that never applied,
becomes zephyr, the theme those installs showed. The update and
rollback steps restore an older version's rewritten theme files only
where they are modified.

appsettings/views.py

@@ -1,7 +1,5 @@
-import os
 from pathlib import Path
 
-import sass
 from admin.decorators import superuser_only
 from django.conf import settings
 from django.contrib import messages
@@ -14,11 +12,8 @@ from operations.utils import BOUNDS
 from appsettings.models import AppSettings
 
 
-def sass_dir_in_project(value):
-    """True if the SASS_DIR value is an existing directory inside the project:
-    the page lists its themes and writes wvc-main.scss into it."""
-    path = (Path(settings.BASE_DIR) / value).resolve()
-    return path.is_dir() and path.is_relative_to(Path(settings.BASE_DIR).resolve())
+# the themes "manage.py build_themes" compiles; base.html links the chosen one
+THEMES_DIR = Path(settings.BASE_DIR, "static", "css", "themes")
 
 
 @superuser_only
@@ -27,87 +22,24 @@ def appsettings(request):
     :param request:
     :return:
     """
-    main_css = "wvc-main.min.css"
-    sass_dir = AppSettings.objects.get(key="SASS_DIR")
     bootstrap_theme = AppSettings.objects.get(key="BOOTSTRAP_THEME")
-    themes_list = []
-    if not sass_dir_in_project(sass_dir.value):
-        messages.error(request, _("SASS directory path must be a directory inside the project: %(dir)s") % {"dir": sass_dir.value})
-    else:
-        try:
-            themes_list = os.listdir(sass_dir.value + "/wvc-themes")
-        except FileNotFoundError as err:
-            messages.error(request, err)
-            addlogmsg(request.user.username, "-", "", err)
+    themes_list = sorted(p.name.removesuffix(".min.css") for p in THEMES_DIR.glob("*.min.css"))
 
-    # Bootstrap settings related with filesystems, because of that they are excluded from other settings
+    # the theme has its own form; the other settings are listed one by one
     appsettings = AppSettings.objects.exclude(
         description__startswith="Bootstrap"
     ).order_by("name")
 
     if request.method == "POST":
-        if "SASS_DIR" in request.POST:
-            if not sass_dir_in_project(request.POST.get("SASS_DIR", "")):
-                messages.error(
-                    request,
-                    _("SASS directory path must be a directory inside the project: %(dir)s")
-                    % {"dir": request.POST.get("SASS_DIR", "")},
-                )
-                return HttpResponseRedirect(request.get_full_path())
-            try:
-                sass_dir.value = request.POST.get("SASS_DIR", "")
-                sass_dir.save()
-
-                msg = _("SASS directory path is changed. Now: %(dir)s") % {
-                    "dir": sass_dir.value
-                }
-                messages.success(request, msg)
-            except Exception as err:
-                msg = err
-                messages.error(request, msg)
-
-            addlogmsg(request.user.username, "-", "", msg)
-            return HttpResponseRedirect(request.get_full_path())
-
         if "BOOTSTRAP_THEME" in request.POST:
             theme = request.POST.get("BOOTSTRAP_THEME", "")
-            # the name goes into @import paths: only the themes in SASS_DIR
             if theme not in themes_list:
                 messages.error(request, _("Unknown theme: %(theme)s") % {"theme": theme})
                 return HttpResponseRedirect(request.get_full_path())
-            scss_var = f"@import '{sass_dir.value}/wvc-themes/{theme}/variables';"
-            # scss_boot = f"@import '{sass_dir.value}/bootstrap/bootstrap.scss';"
-            scss_boot = f"@import '{sass_dir.value}/bootstrap-overrides.scss';"
-            scss_bootswatch = (
-                f"@import '{sass_dir.value}/wvc-themes/{theme}/bootswatch';"
-            )
-
-            try:
-                with open(sass_dir.value + "/wvc-main.scss", "w") as main:
-                    main.write(
-                        scss_var + "\n" + scss_boot + "\n" + scss_bootswatch + "\n"
-                    )
-
-                css_compressed = sass.compile(
-                    string=scss_var + "\n" + scss_boot + "\n" + scss_bootswatch,
-                    output_style="compressed",
-                )
-                with open("static/css/" + main_css, "w") as css:
-                    css.write(css_compressed)
-                # nginx serves the collected copy; collectstatic later takes it from static/
-                served = Path(settings.STATIC_ROOT or "", "css")
-                if settings.STATIC_ROOT and served.is_dir():
-                    (served / main_css).write_text(css_compressed)
-
-                bootstrap_theme.value = theme
-                bootstrap_theme.save()
-
-                msg = _("Theme is changed. Now: %(theme)s") % {"theme": theme}
-                messages.success(request, msg)
-            except Exception as err:
-                msg = err
-                messages.error(request, msg)
-
+            bootstrap_theme.value = theme
+            bootstrap_theme.save()
+            msg = _("Theme is changed. Now: %(theme)s") % {"theme": theme}
+            messages.success(request, msg)
             addlogmsg(request.user.username, "-", "", msg)
             return HttpResponseRedirect(request.get_full_path())