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_accept_invite.py

@@ -1,4 +1,4 @@
-from flask import url_for
+from flask import url_for, session
 
 from bs4 import BeautifulSoup
 
@@ -372,8 +372,8 @@ def test_new_invited_user_verifies_and_added_to_service(app_,
                 mock_add_user_to_service.assert_called_with(data['service'], new_user_id, expected_permissions)
                 mock_accept_invite.assert_called_with(data['service'], sample_invite['id'])
                 mock_check_verify_code.assert_called_once_with(new_user_id, '12345', 'sms')
+                assert service_one['id'] == session['service_id']
 
             raw_html = response.data.decode('utf-8')
             page = BeautifulSoup(raw_html, 'html.parser')
             element = page.find('h2').text == 'Trial mode'
-            assert service_one['id'] in raw_html