Add class based login required

[PabloLefort]

Use of login_required as a method decorator

[das-g]

Do any parts of the Django Girls tutorial or of the Django Girls tutorial extensions already use class-based views? (I couldn't find any.) If not, I'd say that this addition is out of scope for the Django Girls tutorial, which has the goal to provide detailed instructions on one way to do things (so that newcomers can follow), not to provide comprehensive documentation for all variants to do stuff. (The latter is what the official Django documentation and other resources are for).

[das-g]

On, and sorry for the long wait to get this reviewed. I've just resolved a trivial merge conflict that has come up since this PR was filed.