FIX: Various watched words improvements

- Client-side censoring fixed for non-chrome browsers. (Regular expression rewritten to avoid lookback)
- Regex generation is now done on the server, to reduce repeated logic, and make it easier to extend in plugins
- Censor tests are moved to ruby, to ensure everything works end-to-end
- If "watched words regular expressions" is enabled, warn the admin when the generated regex is invalid

app/models/admin_dashboard_data.rb

@@ -96,7 +96,7 @@ class AdminDashboardData
                       :image_magick_check, :failing_emails_check,
                       :subfolder_ends_in_slash_check,
                       :pop3_polling_configuration, :email_polling_errored_recently,
-                      :out_of_date_themes, :unreachable_themes
+                      :out_of_date_themes, :unreachable_themes, :watched_words_check
 
     add_problem_check do
       sidekiq_check || queue_size_check
@@ -224,6 +224,17 @@ class AdminDashboardData
     I18n.t('dashboard.force_https_warning', base_path: Discourse.base_path) unless SiteSetting.force_https
   end
 
+  def watched_words_check
+    WatchedWord.actions.keys.each do |action|
+      begin
+        WordWatcher.word_matcher_regexp(action, raise_errors: true)
+      rescue RegexpError => e
+        return I18n.t('dashboard.watched_word_regexp_error', base_path: Discourse.base_path, action: action)
+      end
+    end
+    nil
+  end
+
   def out_of_date_themes
     old_themes = RemoteTheme.out_of_date_themes
     return unless old_themes.present?