API: Memoize the current_user so that the sudo can work properly

This fixes the SUDO feature of the API, which was broken in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7615/diffs#note_19793649.

Note: We will need to backport this fix to 8.13 and 8.12 as well.

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

Am I overlooking something regarding how the SUDO feature is supposed to work?

Closes #25482 (closed)