UX: Add notice when watched words are regexes (#13493)

There is a big difference between regular watched words and regular
expressions and this has been confusing in the past. This notice adds
an explanation.

This commit also reorganizes the code of the test modal.

app/assets/javascripts/admin/addon/templates/watched-words-action.hbs

@@ -26,6 +26,10 @@
 
 <p class="about">{{actionDescription}}</p>
 
+{{#if siteSettings.watched_words_regular_expressions}}
+  <p>{{html-safe (i18n "admin.watched_words.regex_warning" basePath=(base-path))}}</p>
+{{/if}}
+
 {{watched-word-form
   actionKey=actionNameKey
   action=(action "recordAdded")