Explore backend store setup

[danschultzer]

One thing that I keep thinking about is the backend store. Ref #392 #386.

Looking at the mix phx.gen.auth proposal PR, they put all tokens in the Ecto DB. I like that there's a single source of truth for the user struct. I would like to investigate how a better backend store could look, and see whether the current setup can be streamlined. Would be interesting to rethink the backend store from an Ecto perspective.

ETS and Mnesia still makes most sense to me. It's easier to keep in line with what NIST seems to argue for as well. Ideally session data isn't persisted, and application restarts will wipe out the state. Not ideal for how most web apps works, but it would be great to explore rolling deployments with Mnesia and no disk storage.

[danschultzer]

Pushing work for this with #392 and #567. Inching towards resolving this.