story tag selector: only match at the beginning of the tag name

makes much more sense when quickly typing in tag names to match the
tag name you're typing, not a tag that has a description that
contains a word with what you're typing

app/assets/javascripts/application.js.erb

@@ -301,6 +301,9 @@ $(document).ready(function() {
   $("#story_tags_a").select2({
     formatSelection: function(what) {
       return what.id;
+    },
+    matcher: function(term, text) {
+      return text.toUpperCase().indexOf(term.toUpperCase()) == 0;
     }
   });