Reduce code duplication between inbox.git and redwood.git.

Summary:
This revision consolidates everthing under inbox.status.* into
inbox.status.sync. In addition, it simplifies the bin/inbox-syncstatus script,
which now shares plenty of code with the admin dashboard.

Addresses https://review.inboxapp.com/T656.

Test Plan: Check if inbox and redwood are working correctly.

Reviewers: emfree, jennie

Reviewed By: jennie

Differential Revision: https://review.inboxapp.com/D863