Change /admin/manage_users to use ajax

[allofthenorthwood]

Right now it uses forms and is slow, so deleting/hiding a lot of users takes forever.

Instead, we should use ajax, so when you click to delete/hide a user, it sends off an ajax request, and you can then move on to the next user. We'd need a new state ('deleting'/'hiding') for when the ajax request in in progress.