Improve user/device view for assigning roles

[s-schumann]

Currently, roles can only be assigned in edit, and only if no relations are added. Otherwise it fails.

Needs to be changed to

• work for new relations to immediately assign roles
• be able to delete relation w/o failure

[s-schumann]

Button on users/devices with "Roles" can be added.

Can open /users/1/roles or /devices/4/roles to edit roles (similar to active).

List all users/devices there and select according roles.

Do not allow add/delete devices there.