Use a "logout_user" function that accords with phx_gen_auth and LiveView docs

[peaceful-james]

See this discussion for more illumination:

https://elixirforum.com/t/how-to-implement-pubsub-with-liveview/35257

The comment from "slouchpie" contains code that I think should supplant the custom PubSub in this repo.

To be clear, this would change "Step 4" of the blog post.

[joaobalsini]

Great, thanks for the information, I will take a look and fix it during the week