fix(appsettings): SASS_DIR must be a directory inside the project

The settings page lists the themes of SASS_DIR and writes wvc-main.scss
into it when the theme changes; the value could be any path, so a
superuser could make the app list and write files anywhere it may
(CodeQL py/path-injection).

SASS_DIR is now accepted only if it resolves to an existing directory
inside the project (the default is dev/scss). A stored value outside it
is not used: the page shows an error and offers no themes, so nothing
is written. Installations that keep their SCSS outside the project
need to move it under it.

The tests fail on the previous code for an absolute path, "../",
a path climbing out and a missing directory, and for a stored value
outside the project.

appsettings/test_theme_name.py

@@ -25,3 +25,32 @@ class ThemeNameTests(TestCase):
                 self.client.post(reverse("appsettings"), {"BOOTSTRAP_THEME": theme})
                 self.assertFalse(os.path.exists(os.path.join(self.sass.name, "wvc-main.scss")))
                 self.assertEqual(AppSettings.objects.get(key="BOOTSTRAP_THEME").value, self.before)
+
+
+class SassDirTests(TestCase):
+    """SASS_DIR must be a directory inside the project: the page lists it and
+    writes wvc-main.scss into it."""
+
+    def setUp(self):
+        self.client.force_login(User.objects.create_superuser("sd-admin", "sd@example.com", "pw"))
+        self.before = AppSettings.objects.get(key="SASS_DIR").value
+
+    def test_a_directory_outside_the_project_is_refused(self):
+        outside = tempfile.TemporaryDirectory()
+        self.addCleanup(outside.cleanup)
+        for value in (outside.name, "/etc", "../", "dev/scss/../../..", "no/such/dir"):
+            with self.subTest(value=value):
+                self.client.post(reverse("appsettings"), {"SASS_DIR": value})
+                self.assertEqual(AppSettings.objects.get(key="SASS_DIR").value, self.before)
+
+    def test_the_project_scss_directory_is_accepted(self):
+        self.client.post(reverse("appsettings"), {"SASS_DIR": "dev/scss"})
+        self.assertEqual(AppSettings.objects.get(key="SASS_DIR").value, "dev/scss")
+
+    def test_a_stored_directory_outside_the_project_is_not_used(self):
+        outside = tempfile.TemporaryDirectory()
+        self.addCleanup(outside.cleanup)
+        os.makedirs(os.path.join(outside.name, "wvc-themes", "flatly"))
+        AppSettings.objects.filter(key="SASS_DIR").update(value=outside.name)
+        self.client.post(reverse("appsettings"), {"BOOTSTRAP_THEME": "flatly"})
+        self.assertFalse(os.path.exists(os.path.join(outside.name, "wvc-main.scss")))