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/posts.js.erb

@@ -535,6 +535,11 @@ Post.vote = function(score, id) {
   });
 }
 
+Post.tag = function(post_id, tags) {
+  const tag_string = (Array.isArray(tags) ? tags.join(" ") : String(tags));
+  Post.update(post_id, { "post[old_tag_string]": "", "post[tag_string]": tag_string });
+}
+
 Post.update = function(post_id, params) {
   Post.notice_update("inc");