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.

.github/workflows/test.yml

@@ -154,12 +154,12 @@ jobs:
           done
 
       - name: Import Compiled Dependencies
-        run: docker exec wvc /srv/webvirtcloud/venv/bin/python3 -c "import libvirt, ldap, lxml.etree, psycopg, sass"
+        run: docker exec wvc /srv/webvirtcloud/venv/bin/python3 -c "import libvirt, ldap, lxml.etree, psycopg"
 
       # Static URLs carry no version: browsers must revalidate them
       - name: Static Files Revalidated
         run: |
-          url=http://127.0.0.1:8080/static/css/wvc-main.min.css
+          url=http://127.0.0.1:8080/static/css/themes/zephyr.min.css
           header() { tr -d '\r' | awk -v h="$1" 'tolower($0) ~ "^" h ":" {sub(/^[^:]*: */, ""); print}'; }
           h=$(curl -s -m 5 -D - -o /dev/null "$url")
           [ "$(echo "$h" | header cache-control)" = no-cache ] || { echo "$h"; exit 1; }
@@ -168,7 +168,7 @@ jobs:
           h=$(curl -s -m 5 -D - -o /dev/null -H "If-None-Match: $etag" "$url")
           echo "$h" | head -1 | grep -q " 304" || { echo "$h"; exit 1; }
           [ "$(echo "$h" | header cache-control)" = no-cache ]
-          docker exec wvc sh -c 'echo "/* changed */" >> /srv/webvirtcloud/staticfiles/css/wvc-main.min.css'
+          docker exec wvc sh -c 'echo "/* changed */" >> /srv/webvirtcloud/staticfiles/css/themes/zephyr.min.css'
           h=$(curl -s -m 5 -D - -o /dev/null -H "If-None-Match: $etag" "$url")
           echo "$h" | head -1 | grep -q " 200" || { echo "$h"; exit 1; }
           new=$(echo "$h" | header etag)