[#884] Updated user approval process

It should be possible to request to join an organisation again, and emails wil be sent out when a user does so.
In addition, the user management page is updated;
- The logged in user is not shown in the table
- Only users of approved organisations (of the logged in user) are shown
- The permissions column has been removed, since we need to re-think permissions (read: implement object-level permissions)

Also, on the my details page, the organisation list shows when a user is not approved for an organisation.