THINK-247

This branch a new developer behaviour of branching per sub-task. As our branches start to implement larger features that require more complex logic this makes sense. However we probably want to start doing rebases rather than merges.

This branch represents the start of the work on invite_tokens, share_tokens. There is significant complexity in the api_controller that I'd love @twinge to take a look at.
Just browsed through the code more and maybe `Invitee` is a subclass of `Visitor`? I'm not familiar with this syntax: `class Invitee < ActiveType::Record[Visitor]`