WIP 239 Conversation Notifications

This PR effectively adds New Conversation Notifications by enhancing the existing message notification system.

Conversation#mailing_list is added to return the list of people to send emails to regarding that Conversation. This list contains the participants (people who have actually authored messages) and if assigned the agent. If unassigned the entire team (Account.people) are added.

So when a new message comes in the email notification list is now based on assigned status:

* ASSIGNED: conversation.participants + conversation.agent - message.person (the author of the new message)
* UN-ASSIGNED: conversation.participants + conversation.account.people - message.person (the author of the new message)

WIP: https://assemblymade.com/helpful/wips/239