Fix #4406: Add "commentary:" metasearch.

Add:

* commentary:true (posts with commentary)
* commentary:false (posts without commentary)
* commentary:translated (posts with translated commentary)
* commentary:untranslated (posts with untranslated commentary)
* commentary:"text" (posts where any commentary field matches "text")

Known issues:

* There's no way to escape the true, false, translated, or
  untranslated keywords to do a literal text search for commentaries
  containing one of these keywords.

* Negated searches may be slow. Using a left outer join instead of a
  subquery would be faster in most cases, but negating it is harder.

app/javascript/src/javascripts/autocomplete.js.erb

@@ -288,6 +288,9 @@ Autocomplete.static_metatags = {
   filetype: [
     "jpg", "png", "gif", "swf", "zip", "webm", "mp4"
   ],
+  commentary: [
+    "true", "false", "translated", "untranslated"
+  ],
   disapproved: Autocomplete.DISAPPROVAL_REASONS
 }