Added a Make Admin button to users.js in Admin directory

Currently, the only way to make a user an admin is to search for the username in admin/groups. This works for applications that use Redis for the database but Mongo doesn't currently support text search in production (or at least it's not recommended). Therefore, it's difficult to make a user an admin using Mongo and it would be helpful to have a button under each user in /admin/users (and next to the ban button) which lets a current admin make another user an admin.  

Even if you are using Redis for a backend, it's still helpful to be able to add and remove someone as an admin from /admin/users folder, and, indeed, see if a particular user belongs to the Admin group from looking at the user page rather than having to search through groups.