Strip replies and signatures from emails

Most emails are cleaned up nicely by `griddler`, but the occasional reply text slips through when someone replies from a strange email client (or speaks another language).

There are also some folks who have fancy signatures and `griddler` makes no attempt to clean those out.

This PR adds GitHub's email parsing secret sause, [`email_reply_parser`](https://github.com/github/email_reply_parser), and scrubs all email bodies one last time before creating an entry.
These tests probably already exist in some form in the `email_reply_parser` gem, so it's probably better to just assert that we're using the gem.

Ideally, I think we'd inject the `EmailReplyParser` dependency and verify that we called the correct method on it.

However, that's more annoying than usual because `griddler` does what I've heard @jferris refer to as "class-based programming". It's asked us for the _class_ we want it to call, rather than the _instantiated object_.

Given that, I think we might want to just `allow(EmailReplyParser).to receive(:parse_reply)` and verify it gets called with the email body when we call `#process`.