allow first user to create a superuser account

[amberrignell]

When the app boots and there are no users in the users table
present them with a setup page requiring password and confirm password.
Then create a superuser, and log them in (don’t prompt for password).

All user accessible routes (i.e. not webhooks) should have a plug added to the pipeline to perform this check and redirect to a 'First Setup' view.

While not explicitly included in this, the consideration for caching should be made.
This can be solved by a process that is added to the process tree on startup which checks the database and stores the result in memory - and the plug that checks if the first setup flow should be rendered will get it's answer from that.

However we solve this, there should be a function exposed from a Lightning.Instance namespace (i.e. Instance.has_superuser?/1)

[amberrignell]

Hey team! Please add your planning poker estimate with ZenHub @stuartc @elias-ba