Merge commit '7335a43b6dac0e82c305ce4dec9db4da114c769e' into glitch-soc/merge-upstream

Conflicts:
- `app/helpers/application_helper.rb`:
  Upstream reworked how CSS classes for the document's body are computed.
  Slight conflict due to glitch-soc's different theming system.
  Updated as upstream did.

spec/helpers/application_helper_spec.rb

@@ -8,7 +8,16 @@ RSpec.describe ApplicationHelper do
       before { helper.extend controller_helpers }
 
       it 'uses the controller body classes in the result' do
-        expect(helper.body_classes).to match(/modal-layout compose-standalone/)
+        expect(helper.body_classes)
+          .to match(/modal-layout compose-standalone/)
+          .and match(/theme-default/)
+      end
+
+      it 'includes values set via content_for' do
+        helper.content_for(:body_classes) { 'admin' }
+
+        expect(helper.body_classes)
+          .to match(/admin/)
       end
 
       private