Improve API for UserFromOmniAuth

* Not needing the overhead of a `Struct`
* `valid_auth?` was redundant.
* `provder == 'github' && provider` was redundant.
Matt Bridges was James Bond in disguise!
Prefer `:github_user_id` session name for accuracy

`:github_user` wasn't really describing the contents of the session. It
was a user id.
Intoducing idea of SignedInUser

`User#logged_in?`, while not a terrible idea, did seem out of place. Use a
`SignedInUser` object in conjunction with delegation to pull this method
out of the `User` class.