Only let users unassign roles they are allowed to assign

[Masterjun3]

Resolves #1690 . It also supports neat disabled attributes for the client view.

[vadosnaprimer]

I suggest making text for disabled list items gray, like on disabled buttons.

[Masterjun3]

[vadosnaprimer]

Perfect. Now I can see if I'm able to set the cleared ones or not.