Add lib for user list features. http://listjs.com/

add a jquery plugin for pagination. https://github.com/josecebe/twbs-pagination/

Turned off the users saved in user-groups. Making '$users' parameter in update_usergroup function default to null

In this commit the users list is redefined using list.js and twbsPagination. Mainly using DOM modification triggered by ajax calls.

The users list template should be able to be accessed in notifications and user-groups. In class-module this can be done.

put a check on the javascript so it only run on post creation/edit

put check on user-groups.js so the javascript only run on user-groups

Made a new separate function for adding and removing user to notification. Revert changes made to save_post_following_users() function to prevent breaking other codes.

Put a total user count in hidden field inside the users list template. This then can be used to initialize users list in notifications.php and user-groups.php. Eliminating the need to do ajax request to initialize the users list.

Split the counts logic to its own separate function for code clarity and performance. The ajax_retrieve_users() which called everytime users page repopulated now use get_users() which doesn't include total_count.

Split the logic for retrieving users count into its own ajax endpoint. The function ajax_retrieve_users_in_usergroups() now use get_users() which is more performant as it does note do total_count. So when doing search by keyword we only do total users calculation by keyword only once, in the beginning.

Add additional comments fro explaining functions