clear session on logout

[Matt Lewellyn]

Generally when logout happens, we'll want the session to be cleared as well. We should put a setting in to make this behavior configurable.

Signal:

    @user_logged_out.connect_via(app)
    def _after_logout_hook(sender, user, **extra):
        session.clear()

Test:

    def test_session_cleared_after_signing_out(self):
        with flask.current_app.test_request_context():
            flask.session['foo'] = 'bar'
            assert 'foo' in list(flask.session)
            logout_user()
            assert len(list(flask.session)) == 0