Rework groups

[BelleNottelling]

Groups for both staff members and clients are a very half-baked idea and they should get reworked (probably rebuilt from scratch) before we hit 1.0.0.

At a minimum, users should be assigned to groups rather than groups being assigned to users as logically this makes the most sense and opens up the ability to have someone in multiple groups.
Doing this would be widely useful for categorizing both clients and staff members and just generally be more flexible.

From specifically a staff member group perspective, they need to be able to have permissions assigned to them to greatly simplify the process of setting up permissions for staff.