Fix another missing-argument typo in webhook.py.
Fix limit and offset parameter handling in API.

Summary:
Fix limit and offset parameter handling in API.

The Lens class doesn't take limit and offset parameters as class attributes;
they need to be passed to the message_query and thread_query methods.

Also allow offset to be applied on explicitly unlimited queries, and
fix an unrelated missing-argument typo in webhook.py.

Test Plan: Manual testing.

Reviewers: mg

Reviewed By: mg

Differential Revision: https://review.inboxapp.com/D139