Add authentication

[amberrignell]

We should be using phoenix_gen_auth, this was a community library that has since been merged into Phoenix since 1.6.

As an unauthenticated user, accessing the web frontend I will get a login screen page. Once I enter my email or username (one input) and correct password, I am redirected to the original address I tried to access.
If I enter my details incorrectly, I am presented with a flash message.

This appears to be a very thorough article:

https://www.leanpanda.com/blog/authentication-and-authorisation-in-phoenix-liveview/

Make sure to skip over the authorisation parts and implement only the parts required to enforce that a user must be logged in to access any page.

[amberrignell]

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