We have decorators that check if a user has access to a Local Site and

to check if a user must be authenticated to access a particular view

(based on siteconfig settings). When working with generic views, it's a

bit annoying to use those, as it requires creating a dispatch() method

and using method_decorator() with each of the decorators.
This change introduces four new mixins that apply these decorators to the

view, simplifying the view's responsibilities:

CheckLocalSiteAccessViewMixin is a convenience around the

@check_local_site decorator
UserProfileRequiredViewMixin is a convenience around the

@valid_prefs_required decorator, and
CheckLoginRequiredViewMixin is a convenience around the

@check_login_required decorator.
LoginRequiredViewMixin is a convenience around the

@login_required decorator.

These will be used in some view conversion work that's being done

separately.

Tested these in the view work I'm doing.
Unit tests pass.