Make get_username in pipeline a functor.

[mpessas]

This allows us to factor out the check for the existence of a username, so that
it is easier to override and customize in subclasses.

One of the reasons that this is needed is that django-userena considers two usernames that differ only in case to be the same, while the default behavior of django-social-auth does not. As a result, userena will fail with a MultipleObjectsReturned exception.

With this change, developers will only have to subclass GetPassword and override just the username_exists method.

[omab]

I'm not in favor of the class with a call(), instead I've added a override option for the user exists check, you will be able to create your pipeline get_username that calls the original one with a function that implements the needed check.

[mpessas]

The thing is, that I was thinking of having more things "configurable", like how a random username would be generated (just nice to have), which would be easier to do with a functor than adding another function as a parameter or in settings.
But ok, since the main problem is solved, thanks!

[omab]

Things being configurable is the purpose of the pipeline, if the default behavior doesn't fit, then it's the time to implement the needed functionality for the project as a new pipeline item.

The main idea is to have small functions that do the work properly. But DSA aims to provide a default basic behavior, not cover all the possible cases or it will end like a big classes collection with lot of overrideable methods for "just in case" scenarios.

Thanks.