Remove sites dependency

Sites is no longer enabled by default since Django 1.6.x, see: https://github.com/django/django/commit/23fc91a6

Sites was only necessary for one remaining location in the code (email confirmation upon email change in `SettingsView`), and this code now uses `get_current_site` to remove that dependency (as is done for the email confirmation done by `SignUpView`).

I think this change means that the `EmailConfirmation.send` method will always receive a `site` keyword argument (so code could be rewritten to depend on receiving a `site` kwarg). I haven't changed the code more extensively because it's possible some users overriding aspects of this package won't be passing a `site` argument, and you may want to continue supporting that.