New config system

Summary:
So I've been pretty unhappy with our config system for a while. Boostrapping redwood off it was the last straw, and here's a first stab at fixing it. In general, I feel like configuration needs to be powerful to remain simple, and that *configuration is code* at the end of the day.

Some things I didn't like about the previous system:

* Massaging booleans from `yes` or `no` to native Python `True` or `False` types. Yuck.
* Expanding paths based on potential relative user directories
* Overriding the entire config file for tests, when really we only change one or two params
* This `config-sample.cfg` stuff, when most/all of the defaults are fine
* The notion of production vs. staging vs. development. It's really premature optimization, given that we have literally zero production machines and nothing running on RDS at the moment.

I had a good conversation with Eben over lunch about how Google uses feature flags to turn on/off specific code paths. I think this might be a better approach, since it will be clear what's actually enabled, and not guessing based on staging/production/etc. One first pass at this is deciding to send emails by the `EMAIL_EXCEPTIONS` configuration value.

I also want to explore using our pluggable modules to override configuration via other modules which might import `inbox`. You can imagine this in a test scenario, or the case of redwood, which uses a lot of shared utilities and database functions.

This commit also breaks out where we're creating the database engine, since we don't want to create it in code that other modules import.

In general, I want to start a discussion about this (hence cc'ing a few of you). The goal here is to figure out something that adds some more flexibility immediately (land this quickly), but gets us on the right track towards a comprehensive solution later.

Test Plan: One tests fails... I'm not sure why.

Reviewers: emfree, kav-ya

Reviewed By: emfree

CC: emfree, kav-ya

Differential Revision: https://review.inboxapp.com/D166