DEV: Use more specific error responses (#9472)

* DEV: Use `render_json_error` (Adds specs for Admin::GroupsController)
* DEV: Use a specific error on blank category slug (Fixes a `render_json_error` warning)
* DEV: Use a specific error on reviewable claim conflict (Fixes a `render_json_error` warning)
* DEV: Use specific errors in Admin::UsersController (Fixes `render_json_error` warnings)
* FIX: PublishedPages error responses
* FIX: TopicsController error responses (There was an issue of two separate `Topic` instances for the same record. This makes sure there's only one up-to-date instance.)

spec/requests/topics_controller_spec.rb

@@ -989,7 +989,7 @@ RSpec.describe TopicsController do
           }
 
           expect(response.status).to eq(422)
-          expect(JSON.parse(response.body)['errors']).to be_present
+          expect(response.parsed_body['errors']).to match_array([/Title is too short/, /Title seems unclear/])
         end
 
         it "returns errors when the rate limit is exceeded" do