Merge commit 'c58967c3bd7b7eee0b284ee2b77a48ed9ce6863e' into glitch-soc/merge-upstream

Conflicts:
- `lib/sanitize_ext/sanitize_config.rb`:
  Conflict due to upstream adding fallback support for MathML Core while
  glitch-soc supported slightly more tags than upstream.
  Added the MathML Core fallback code to glitch-soc.
- `spec/lib/sanitize/config_spec.rb`:
  Ditto.

spec/requests/api/v1/lists/accounts_spec.rb

@@ -95,7 +95,7 @@ RSpec.describe 'Accounts' do
       it 'does not add the account to the list', :aggregate_failures do
         subject
 
-        expect(response).to have_http_status(404)
+        expect(response).to have_http_status(422)
         expect(response.content_type)
           .to start_with('application/json')
         expect(list.accounts).to_not include(bob)