Remove userId from MembershipResource interface

[spolu]

Description

We cut the MembershipResource with userId(s) (number) in its interface which breaks our invariants.

This PR refactors parts of the code to use UserType everywhere.

Behavior change:

• When rendering the authors of an assistant we don't filter along membership, showing authors that may have been revoked.
• When deleting memberships of a workspace (GDPR scrubbing) the logic is changed to properly delete users if they only belonged to that workspace (otherwise they would be dangling with no membership, I think that's what we want) (r? @PopDaph)

Risk

Risky as it touches many parts of the code, will run tests on all touched parts (esp. agent author display)

Test plan:

• poke revoke
• Login
• recent authors display
• Invite user / revoke user
• general use of the app

Deploy Plan

• deploy front

[fontanierh]

Looking good