Implement user role tree

This PR implements a user role tree.

Features:
- New View called `prinicpal_role_tree` (Principal because it would be possible to search for groups later.)
- Search for users by fullname and username using select2 jquery widget.
- Build a tree based on the local roles of the user
- Resolve group memberships

Missing:
- [x] Upgrade Step
- [x] Remove old table based tree view
- [x] Add further actions to remove / copy roles of the chosen user.
- [x] Display if local roles are not acquired. 
- [x] CSS

Nice to have:
- [ ] Use select2 widget for copy / remove user roles too

TODO:
- [x] Performance test with ldap