Allow thread tags to be modified either by name or public id.

Summary:
Previously we would only check tag names, which works fine for things
like the 'inbox' tag, where the tag name and public_id values generally
coincide. But this is not good for user-created tags, where the public_id is
constant but the name may actually change.

Fixes T467.

Test Plan: Regression test added.

Reviewers: charles

Reviewed By: charles

Subscribers: bengotow

Maniphest Tasks: T467

Differential Revision: https://review.inboxapp.com/D480