Use get_current_site(), instead of Site.get_current()

django-user-accounts requires the sites framework to be in use, which makes things awkward for those who would prefer to use `RequestSite` (e.g. we're using django-hosts).

This adds a new `site` parameter to email-sending methods, which is passed through from the view. The view uses `get_current_site(request)`, instead of `Site.get_current()`, as the more generic (and `RequestSite`-aware) way to get a `Site`-like object.