Define Inbox Conversation

[jazzz]

Clients must be able to retrieve an initial message from a contact, in order to create a conversation.

• This pathway needs to be able to receive messages encrypted from previously unknown contacts.
• Allow clients to receive invites to conversations
• This should be framed as a conversation type

[fryorcraken]

@jazzz can you move this to the repo that contains the code so we can only have milestones and deliverable in pm repo?