Support the 'attachment' tag on threads.

Summary:
If at least one message on a thread has attachments, add the 'attachment'
tag to that thread.

Note that this code doesn't properly handle the case in which you create a
draft with attachments, then update it to a draft that has no attachments.
Since we're in the process of redoing draft versioning, I propose that we make
a bug to fix this after the new drafts stuff lands.

Fixes T323.

Test Plan:
Run migration script, and run a fresh sync. Each time, check
responses to /n/ns_id/threads?tag=attachment. Also update drafts unit
tests.

Reviewers: kav-ya

Reviewed By: kav-ya

Maniphest Tasks: T323

Differential Revision: https://review.inboxapp.com/D363