jquery.wikiEditor.preview: Clean up

* Rename 'collection' to 'loadmodules'.
* Consistently close parentheses and brackets at the same
  indentation level as they were opened.
* Use $.ajax directly instead of $.post, especially the fourth
  argument to $.post makes it hard to read.
* Use promise interface of $.ajax and $.post instead of the
  (deprecated) callback parameter.

Change-Id: I48924c9a1ce29829f8994be05c46d5563b2c25d6