add login option: stay_logged_in

As proposed in issue #354; it adds an attribute max_age
to mediagoblin.tools.session.Session  that is passed to
response.set_cookie;  max_age is set to 30  days if the
checkbox is selected

mediagoblin/plugins/basic_auth/forms.py

@@ -41,3 +41,5 @@ class LoginForm(wtforms.Form):
          normalize_user_or_email_field()])
     password = wtforms.PasswordField(
         _('Password'))
+    stay_logged_in = wtforms.BooleanField(
+        _('Stay logged in'))