Improve english in README.md
Change method name to `redirect_if_logged_in()`

This name is more descriptive of what the method does. It returns a
redirect response hash if user is logged in.
Make redirect_if_logged_in() private

There is no need for it to be public.
Encapsulate is logged in logic into is_logged_in()

It's always better to encapsulate so logic becomes more readable and
composable (easier to reuse).
Encapsulate login and redirect response into it's own method
Encapsulate rustic data model building
Whitespace
Update LoginDataAction so that it provides values in it's result

This makes our LoginController tidier and keeps the data logic where it
should be.
Rename method to redirect_if_not_logged_in()

This name better describes what and why this function does what it does.
Encapsulate is_logged_in() logic