Class-based Login, Logout and Callback views, plus successful_login overridable method

[duoi]

As per #175.

These changes seem to make it fully Django 2.1 compatible, and running the tests locally also show no issues with Python 3.7. May be worth updating tox or the docs to reflect this if the tests are passing on your end.

The only major change that might affect users is that cas_ng_login now has to be the name of the login URL, in line with what is already in the documentation. Previously we were just calling reverse() on the function itself. But since we have class-based views we can't do that anymore (as far as I'm aware).

I'm also passing the entire request object to successful_login now instead of just user. I imagine this will give the most possible flexibility if people are passing in additional flags from the CAS server or have some middleware or something.

[mingchen]

Could you document this change in README.md and its impact?

[duoi]

@mingchen done

If you want to add something to release notes:

The `name` value given in the documentation for the `login` view ("`cas_ng_login`") is now required. Historically we were using function-based views and were able to pass that to the `reverse()` function, but we can no longer do that. This change only affects users of the `middleware` at the moment, so if you are not using it then it might not affect you. This is a breaking change if you have given the `login` view a different name than what is specified in the documentation.