Give admins option to test if a theme loads correctly

If a theme is uploaded incorrectly or has errors in it, users can still
select the theme but it will cause a 500 error on every page, making the
app unusable and also making it impossible for them to switch to a
functional theme.

A better fix would be to fail gracefully, but in lieu of that, this will
at least let admins confirm if a theme is broken safely.

bookwyrm/views/__init__.py

@@ -30,7 +30,7 @@ from .admin.reports import (
     moderator_delete_user,
 )
 from .admin.site import Site, Registration, RegistrationLimited
-from .admin.themes import Themes, delete_theme
+from .admin.themes import Themes, delete_theme, test_theme
 from .admin.user_admin import UserAdmin, UserAdminList, ActivateUserAdmin
 
 # user preferences

bookwyrm/views/admin/themes.py

@@ -6,6 +6,8 @@ from django.utils.decorators import method_decorator
 from django.views import View
 from django.views.decorators.http import require_POST
 
+from sass_processor.processor import sass_processor
+
 from bookwyrm import forms, models
 
 
@@ -40,6 +42,7 @@ class Themes(View):
 def get_view_data():
     """data for view"""
     return {
+        "broken_theme": models.Theme.objects.filter(loads=False).exists(),
         "themes": models.Theme.objects.all(),
         "theme_form": forms.ThemeForm(),
     }
@@ -52,3 +55,20 @@ def delete_theme(request, theme_id):
     """Remove a theme"""
     get_object_or_404(models.Theme, id=theme_id).delete()
     return redirect("settings-themes")
+
+
+@require_POST
+@permission_required("bookwyrm.system_administration", raise_exception=True)
+# pylint: disable=unused-argument
+def test_theme(request, theme_id):
+    """Remove a theme"""
+    theme = get_object_or_404(models.Theme, id=theme_id)
+
+    try:
+        sass_processor(theme.path)
+        theme.loads = True
+    except Exception:  # pylint: disable=broad-except
+        theme.loads = False
+
+    theme.save()
+    return redirect("settings-themes")