Automatically start syncs for newly-registered accounts.

Summary:
In inbox/mailsync/service.py, poll for accounts that have been registered but
don't have syncs started, and start them.

Also remove support for the 'startall/stopall' functionality in bin/inbox-sync,
which is not that useful and adds unnecessary complexity to the normal
start/stop sync implementation.

Test Plan:
Auth a new account and check that the sync starts; run
`bin/inbox-sync start <email_address>` and `bin/inbox-sync stop <email_address>`.

Reviewers: kav-ya, spang

Reviewed By: kav-ya, spang

Subscribers: kav-ya

Differential Revision: https://review.inboxapp.com/D237