WIP 119 Account Specific Email

**Not ready to merge yet. Would love some feedback on the approach**

Per: https://assemblymade.com/helpful/wips/119

This is a pretty big PR:

- Setup some backend stuff to enable replies to Helpful notification emails (account_slug+conversation_number@helpful.io).

- Use Mail::Address to parse email addresses, this lets us get access to both the name and address which we can use for the person object.

- Remove MessageMailer and move to ConversationMailer. This better tracks with the behavior I was thinking about, there are two different types of emails:
    1. An email is sent to the customer who created the support request
    2. An email is sent to everyone in the conversation each time a new message is created (except the person who created the message)

Still todo:

- [ ] Refactor code in ConversationMailer, there is a lot of duplication between the the two methods.
- [ ] Fix styling in HTML emails (I need some help with this)
- [ ] Figure out how the web app should post new messages (MessagesController or via API::MessagesController)