[keyserver] Merge updated and legacy multimediaMessageCreationResponder(...)

Summary:
Addresses @tomek's feedback here: https://phab.comm.dev/D5127#150659

Basically cuts a bunch of redundant code and just branches where the two request payloads vary (mediaIDs vs mediaMessageContents)

Thought it was simpler to put up a refactor diff rather than go back and change D5127, hope that's okay.

Depends on D5127

Test Plan: Things continue to work as expected. Set breakpoints and observed correct values at each step.

Reviewers: tomek, abosh, marcin

Reviewed By: tomek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D5191