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

spec/services/word_watcher_spec.rb

@@ -24,7 +24,7 @@ describe WordWatcher do
       it "is correct when watched_words_regular_expressions = false" do
         SiteSetting.watched_words_regular_expressions = false
         regexp = WordWatcher.word_matcher_regexp(:block)
-        expect(regexp.inspect).to eq("/(?<!\\w)((#{word1}|#{word2}))(?!\\w)/i")
+        expect(regexp.inspect).to eq("/(?:\\W|^)(#{word1}|#{word2})(?=\\W|$)/i")
       end
     end
   end