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/published_pages_controller_spec.rb

@@ -114,6 +114,7 @@ RSpec.describe PublishedPagesController do
           PublishedPage.create!(slug: 'i-hate-salt', topic: Fabricate(:topic))
           put "/pub/by-topic/#{topic.id}.json", params: { published_page: { slug: 'i-hate-salt' } }
           expect(response).not_to be_successful
+          expect(response.parsed_body['errors']).to eq(['Slug has already been taken'])
         end
 
         it "returns an error if the topic already has been published" do