Document architecture

[danillouz]

Current design (minus error handling, which depends on #11)

[danillouz]

User invites:

[danillouz]

User invites v2:

[danillouz]

The user-invites (FIFO) queue has been removed for now, and the queue's Lambda trigger has been repurposed to a SNS trigger, subscribing to the new-invite SNS topic.

For more info see: account issue no. 7 and infra issue no. 17