[lib] Implement messageContentForServerDB for messageTypes.MULTIMEDIA

Summary:
The messageContentForServerDB(...) function determines what gets put in the content column of the messages table.

The current content column for media messages is a "stringified" list of uploadIDs. However, the content column is never used after it's been stored in the DB. Rather, messages are joined with corresponding uploads via the container column of the uploads table. Therefore this change isn't "destructive."

Depends on D5195

Test Plan:
Rows in the messages and uploads column appear as expected:

Reviewers: abosh, tomek, marcin

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D5197