Fix email replies

This fixes two bugs:

1. Mailgun recipient variables weren't matching up with the To fields because the recipient keys were email addresses and the To fields included display names (`zach@hackerschool.com` vs. `Zach <zach@hackerschool.com>`). The fix for this (for now) was to go back to just email addresses in the To fields.

2. Replying didn't work depending on your email client because `reply_info` was case-sensitive (base 64 encoded), but email addresses are not. Gmail would keep the case, but Airmail downcased everything, meaning I couldn't reply. The fix for this is to switch to a case-insensitive `ReplyInfoVerifier` that doesn't base 64 encode the serialized data.
Kind of. The whole point of this is that it's case-insensitive (because it goes in email addresses). Except that it's not _actually_ case-insensitive, so we just normalize on lower case.