[DEPENDENCIES] Integrate exception logging with Sentry.

Summary:
For large deployments, exceptions over email get extremely unwieldy.

This commit also fixes up the email code to use `config.get_required()`
and moves the structlog configuration as far up in inbox/log.py as we
can (it's strange to have code that runs on import way down at the
bottom).

There is some known weirdness with integrating raven's logger with structlog
(see for example https://github.com/getsentry/raven-python/issues/377), but
we can look more into weeding out extra errors later---let's first see what
things look like in practice with Sentry's default verbose logging.

We may also want to hook into the logging infrastructure directly later and
report on more errors than just mailsync uncaught exceptions.

Test Plan: Manual triggering of exceptions.

Reviewers: kav-ya

Reviewed By: kav-ya

Differential Revision: https://review.inboxapp.com/D273