UX: require confirmation of revoking permissions on
repos/user groups/repo groups. We don't want people to
click on this accidentally.

UserGroup on UserGroup permissions implementation.
- user group can have another usergroup assigned
- it's impossible to assign usergroup on the same usergroup

consistent handling of grant/revoke of permissions widgets
- use 1 method for users and for users groups
- use common JS for ajax revoke
- cleanup code

- Manage User’s Groups: create, delete, rename, add/remove users inside.
by user group admin.
In this case, a user's group can be owned by several people thru an owner user's group.

Some refactoring of naming, permission handling logic.
- remove some code duplicity as well as inconsistent naming