Remove WizardView.get_wizard_name()

[Bradley Ayers <bradley.ayers@…>]

I propose removing WizardView.get_wizard_name. I think this is a good idea
because currently it seems like get_wizard_name is essentially just the current implementation of
get_prefix. get_wizard_name isn't used anywhere else, and adds the burden of
being an API that would need to be maintained. Essentially this would just mean moving
the contents of get_wizard_name into get_prefix.

I also propose changing WizardView.get_prefix to accept `request, *args,
kwargs`. This would allow for greater flexibility in how the prefix is
determined.

My current use-case is that I want to be able to have multiple incomplete wizards
in progress at the same time. I then want to have the wizard prefix in the URL,
so that a user is able to navigate to the different incomplete instances of the wizard
that they're currently completing. I've achieved this for my situation by reimplementing WizardView.dispatch and WizardView.get_prefix, but my above suggestion would simplify this.