Make admin api redactions use the requester to send the redaction

[morguldir]

Pull Request Checklist

• Pull request is based on the develop branch
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[anoadragon453]

Thanks for catching and fixing this! I'm surprised this wasn't covered by a unit test, which means it could break again in future.

Would you be willing to write a test (likely here) that tests redacting events from a remote user? You can use the inject_event test utility to create and send events from remote users (usage example).

[H-Shay]

@morguldir thanks for catching this and fixing it. @anoadragon453 this should be fixed - if you need a test to merge this PR, I've added one here.

[anoadragon453]

This change LGTM. I've also confirmed that #18043 passes only once the changes in this PR are applied.

Thank you!

(Note: CI failures here are unrelated to this PR.)