Role Request: Mutually exclusive roles & role requirements

[rubellyte]

Two suggestions in this one:

1. Making groups of mutually exclusive roles - if you select one in the group, it will remove any of the other ones you have in the same group.

2. Only allowing the requesting of a role if a predicate is satisfied - namely, having another role already.

[GTG3000]

Implemented 1, working on 2