Tag scripts: drop [if ...] and [reset] syntax (fix #3807, #3773)

Fixes #3807. Drops the TagScript module entirely and replaces it with
a simple call to Post.tag.

Simultaneously fixes #3773 as a consequence of removing TagScript.run.
Post.update already updates data-tags by itself, so there was no need
for TagScript.run to update data-tags as well.

app/javascript/src/javascripts/utility.js

@@ -99,16 +99,6 @@ Utility.without = function(array, element) {
   return temp;
 }
 
-Utility.reject = function(array, f) {
-  var filtered = [];
-  $.each(array, function(i, x) {
-    if (!f(x)) {
-      filtered.push(x);
-    }
-  });
-  return filtered;
-}
-
 Utility.regexp_escape = function(string) {
   return string.replace(/([.?*+^$[\]\\(){}|-])/g, "\\$1");
 }