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
e64594ac44