Merge Access and Invitations feature branch

Here it is!

After lots of hard work (thanks everyone!) and almost 200 commits, this is ready to be merged into `master`. We will then deploy to `staging` for final testing and after that, on to `prod`!

@jebeck Could you confirm that you don't have any ongoing work in `master` that this merge would impact? After you give us the green light, we'll merge this branch. Once that's done, remember to pull and `npm install` before starting any work on Blip again! :)

Cheers