Switch from `datetime` to `firestore.SERVER_TIMESTAMP`

[benjaminkostiuk]

For some reason in production with AWS pushing datetime objects to firestore causes them to be incorrectly parsed. Using SERVER_TIMESTAMP works and has been used as a workaround for now.

Ideally we root cause and figure out what exactly is the issue. Or we can just switch over wherever we push to firebase to use SERVER_TIMESTAMP instead of the datetime.now(timezone.utc) call. Dunno what to do about a specific time.