[placeholder] Update edits of user groups

[tcompa]

(not yet implemented in the backend, I will update this issue when it is)

In the current status:

• We cannot remove users from a user group
• Adding users to a user groups is meant to also work for the "bulk" case, even though we only add one user at a time in the current fractal-web UI.

As soon as fractal-analytics-platform/fractal-server#1841 (starting from fractal-analytics-platform/fractal-server#1841 (comment)) is implemented and released, two things will change:

• The current way of adding users will be deprecated
• There will be two more atomic endpoints that add/remove a single user and then return the updated group object:

POST /group/{group_id}/add-user/{user_id}/
POST /group/{group_id}/remove-user/{user_id}/

Relevant PR:

• Allow deleting users from groups fractal-server#2101

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Side-note: we are leaving the possibility of bulk user addition/removal for the future, and it may eventually be part of POST /auth/group/{user_group_id}/add-users-bulk/ and POST /auth/group/{user_group_id}/remove-users-bulk/.