When not using the Django sites framework, you get a RuntimeError
because it is not installed, even if you set DJANGO_MESSAGES_NOTIFY to
False.

This can be avoided by importing the site model only when necessary.

By default, message list is not paginated, but now one can enable pagination
by setting ``DJANGO_MESSAGES_PAGE_LENGTH`` to a positive integer.

Pagination is disabled if this setting variable is not defined or set to -1.

Paginator.get_page() is only available in django >= 2.0, let's use
Paginator.page() instead.