[SCHEMA] Begin adding tag support to API.

Summary:
 * Rename InternalTag to UserTag, and add another table SystemTag for reserved,
   Inbox-managed tags that don't map to a backend folder (e.g., "sending",
   files")

 * Support folder name aliasing, so that we can handle Gmail's messy
   labels-aka-folders abstraction. I.e., treat "Sent" and "[Gmail]/Sent Mail"
   as the same folder.

 * Make the relationship between threads and tags many-to-many.

 * Add exposed_name and public_id fields to the Folder table to store the names
   we use when when exposing the folder as a tag.

 * Support tag as a query parameter in API queries. Current limitations:
   - Can only match on one tag at a time.
   - Can't match on public_id of user-created tag.

Test Plan: Ha! Not really. I did play with it a bit though.

Reviewers: spang, mg

Reviewed By: spang

CC: bengotow

Differential Revision: https://review.inboxapp.com/D148