[SCHEMA] Add support for per-message unread marking, and the unseen tag.

Summary:
 * Don't store 'user_mutable' as a column in the tags table. Instead, implement
   properties tag.user_addable and tag.user_removable (these are distinct,
   since the 'unseen' tag may be removed but not added).

 * Add API support for setting a message's unread attribute.

 * Automatically remove the unseen tag when the unread tag is removed.

 * Update documentation.

Test Plan: Unit tests updated.

Reviewers: spang

Reviewed By: spang

Differential Revision: https://review.inboxapp.com/D195