Add support for excluding object types from delta sync API.

Summary:
This allows clients who don't care about, say, calendar events to limit
the changeset they get from the /delta and /delta/streaming endpoints, for
better performance.

Test Plan: Unit tests updated.

Reviewers: khamidou

Subscribers: bengotow

Differential Revision: https://review.inboxapp.com/D757
Don't strip quoted text.

Summary:
Too unreliable for our use case without significant changes to Talon's
internals.

Test Plan: Run message-parsing unit tests.

Reviewers: spang, mg

Reviewed By: mg

Differential Revision: https://review.inboxapp.com/D761