autocomplete: trigger completion for metatags after typing colon (#3558)

When completing metatags (e.g. `rating:safe`), show completions
immediately after typing the colon (`rating:`), instead of after the
first letter (`rating:s`).

app/assets/javascripts/autocomplete.js.erb

@@ -115,11 +115,6 @@
         var metatag = query.metatag;
         var term = query.term;
 
-        if (!term) {
-          this.close();
-          return;
-        }
-
         switch(metatag) {
         case "md5":
         case "width":