[keyserver] Modify legacyMultimediaMessageCreationResponder to handle new payload

Summary:
This is a temporary change on the keyserver side that keeps things working as they are when client hits create_multimedia_message with the new payload.

We're basically translating mediaMessageContents back to mediaIDs by pulling out the uploadID property.

This ensures that the client continues working as normal with the new payload while we're still working on implementing the "new" multimediaMessageCreationResponder.

Depends on D5061

Test Plan:
Able to send image(s) on web and native as expected.

Set breakpoints at various points throughout the media message flow and observed that values were as expected at each point.

Reviewers: abosh, tomek, marcin

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D5062