Update imapuid index name to reflect latest columns.

No migration included here because foreign key constraints already
caused mysql to DTRT, albeit without the rename.
Remove User, SharedFolder, and UserSession from models and API

Summary: spring cleaning

Test Plan: all tests pass

Reviewers: spang

Reviewed By: spang

Differential Revision: https://review.inboxapp.com/D144
Rename the reverse ImapUid/Folder relationship to 'imapuids' (from 'uids').
We do this so as to support a similar EASUid/Folder relationship.
Implement audit timestamps on all tables.

Note that this initial support does not make any of our code
delete-aware yet (exclude deleted objects from results or mark objects
as deleted).
Automatically filter soft deletes.

TODO:
- figure out how to modify my MapperOption so it can be applied BEFORE filters
- subclass session to provide our own query method?