Reply to messages via email

[joemasilotti]

This PR implements inbound email processing for replying to messages.

Closes #423.

To-dos:

• Implement Action Mailbox to receive inbound emails
• Create a new message when an inbound email is received in reply to a notification email
• Note in the message email that you can reply to the email directly
• Properly obfuscate the "from" field so someone can't guess it and send a message to a conversation they aren't a part of
• Mark the conversation's notifications as read continued via email
• Run through Businesses::Permission to ensure user is permitted to send the message
• Ensure conversation isn't blocked
• Configure Postmark and production settings

Questions/concerns:

• Is mail.from "safe" - can we trust it actually is that person?
• Is reply-to "conversations-#{id}@" the right approach - or should we explore a header?
• What about multipart (HTML) email bodies?
• Routing all: :messages isn't sustainable.
• Do we need to handle "reply above this line"? Not until I use it in production for a bit.

Pull request checklist

• My code contains tests covering the code I modified
• I linted and tested the project with bin/check
• I added significant changes and product updates to the changelog Not until I remove the feature flag

[joemasilotti]

@fig mentioned on the stream:

Stripping out signature blocks —

​I'm not sure how reliable it is, but properly configured email client should add a signature block identifier (2 dashes, a space, carriage return, new line) which we can look for and remove all after.