Actually support EAS tags.

Summary:
EAS used Folder.canonical_name to store the eas_folder_id - a unique identifier
from the server, required to perform any action at all.
At the time it was implemented, using the canonical_name provided
the ease of filtering on both name, eas_folder_id thereby preventing
duplicate entry errors we were seeing (the specific issue then was the user had
two different folders of the same name).

This doesn't fare so well for EAS tags.
Therefore introduce an extra 'identifier' column.

I looked into a couple of ways to avoid a schema change
but things like the inability to override SqlAlchemy validators
in subclasses makes this the logically cleanest way to do it.

Test Plan:
Visual inspection of an EAS account.
Sync runs, test passes.

Reviewers: emfree

Reviewed By: emfree

Differential Revision: https://review.inboxapp.com/D468