[SCHEMA] Fixes T207: EAS Integrity Error

Summary:
EAS folders are uniquely determined by a folder_name and an eas_folder_id.
We pass in the eas_folder_id as the canonical name since Folder.find_or_create()
queries on both name, canonical_name before creating a Folder.

The included migration:
1. Changes the unique constraint on Folder from (account_id, name) to (account_id, name, canonical_name)
2. Adds a unique constraint on EASFolderSyncStatus for the (account_id, eas_folder_id) relation.

Test Plan: Syncs (EAS, other) run as before.

Reviewers: spang

Differential Revision: https://review.inboxapp.com/D275
[DEPENDENCIES] lxml (for talon).

Also fix import in tests/imap/network/test_actions_syncback.py