Update Invitations API to accept users

What does this MR do and why?

Adds ability for the invitations API and the service it uses to accept user_id as well as email. Can send both as well.

This is being done to further #350999 (closed) and allow us to use only the invitations API in the invite modal.

Larger picture for this can be seen in !80733 (merged)

invitations API was introduced a little over a year ago for primary use on the invitations modal. While it is a public API, most use is from that modal.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #350999 (closed)