Merge branch '21983-member-add_user-doesn-t-detect-existing-members-that-have-requested-access' into 'master'

Resolve "`Member.add_user`doesn't detect existing members that have requested access"

## What does this MR do?

This merge request handle the case when an access requester is added to a group or project (via the members page or the API).

In `Member.add_user`, if an access requester already exists, we simply accept their request (and set the `created_by`, `access_level` and `expires_at` attributes if given).

## Are there points in the code the reviewer needs to double check?

I've taken the opportunity to cleanup the whole `{Group,Project}Member.add_user*` methods since it was quite a mess.

## What are the relevant issue numbers?

Closes #21983

See merge request !6393

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