Allow both addition and removal of groups from expression policies

**Describe the solution you'd like**
Allow expression policies to add and remove the current user from groups.

**Describe alternatives you've considered**
None available to my knowledge.

**Additional context**
Currently you can add a __new__ users to groups using dynamic groups (https://goauthentik.io/docs/flow/stages/user_write#dynamic-groups).
Sadly there's two missing features:
- You can't do it for existing users (Would love to update roles when users log in)
- You can't remove roles