Make new service current service on first use

When you add a new service, it’s probably the one you want to do stuff
with.

When you get invited, the service you’ve been invited to is probably the
one you want to use.

This commit adds the ID of the new service or service you’ve been
invited to to the session.

tests/app/main/views/test_add_service.py

@@ -1,4 +1,4 @@
-from flask import url_for
+from flask import url_for, session
 
 import app
 
@@ -25,15 +25,18 @@ def test_should_add_service_and_redirect_to_next_page(app_,
             response = client.post(
                 url_for('main.add_service'),
                 data={'name': 'testing the post'})
+            assert mock_get_services.called
+            mock_create_service.assert_called_once_with(
+                service_name='testing the post',
+                active=False,
+                message_limit=app_.config['DEFAULT_SERVICE_LIMIT'],
+                restricted=True,
+                user_id=api_user_active.id,
+                email_from='testing.the.post'
+            )
+            assert session['service_id'] == 101
             assert response.status_code == 302
             assert response.location == url_for('main.tour', page=1, _external=True)
-            assert mock_get_services.called
-            mock_create_service.asset_called_once_with(service_name='testing the post',
-                                                       active=False,
-                                                       limit=app_.config['DEFAULT_SERVICE_LIMIT'],
-                                                       restricted=True,
-                                                       user_id=api_user_active.id,
-                                                       email_from='testing.the.post')
 
 
 def test_should_return_form_errors_when_service_name_is_empty(app_,