Make selectors for user management page more specific

* Assigned jQuery selector results to variables for speed.
* Moved all .attr('data') calls to .data() calls.
* Added :data(uid) selector.
* Split the multiselect checkhandlers into separate functions.
* De-duped click event on password change/img.
* Used fluent interface on jQuery object for event binding.
* Removed redundant tag names in class selectors.
* Fixed incorrect date display for last login.
* Added Userlist.has() to search for uid.
* Fixes issue where "admin" group would disappear form dropdowns after search results (not caused in this branch, as it turns out)
* General code convention compliance fixes.

Please review @blizzz @PVince81 @raghunayyar