JSDocs - Callback parameters

* `callback` is documented as `Standard callback function` everywhere
* `callback.err` is documented as `An error that occurred, if any` everywhere
Folders - Cleanup

 * Moved `getFoldersForContent` and `authzPrincipalsToFolder` into `authz.js`
 * Cleaned up `previews.js`
 * Cleaned up REST docs
 * Cleaned up search
 * Cleaned up test utility
 * Cleaned up search
 * Expanded on search tests