Make Member#add_user set access_level for requesters

## What does this MR do?

This is a backport of https://gitlab.com/gitlab-org/gitlab-ee/commit/c902d8d48409a4d46f7669fc5fc85b9ba86e71d6 from gitlab-org/gitlab-ee!744.

## Why was this MR needed?

This change is only used in EE right now, but there's no reason why it wouldn't be part of CE.

See merge request !6649