Implement frontend for group management

[ChristianSi]

Allow users with the required Role to assign groups to personas or groups, and to unassign them.

[fisx]

For (US2)[https://github.com/liqd/thentos/milestones/US2], we should aim for something simple: When creating a new persona, the user is presented with a list of all processes and can select one. This assignment will never change. Something like that.

For anything less trivial, we need more detailed user stories.