refs https://github.com/TryGhost/Team/issues/694
refs https://linear.app/tryghost/issue/CORE-10/tackle-integrationsjs
- The controller code is not meant to contain complex business logic.
- Added a test case checking 'PUT' endpoint for integrations to ensure
proper 'NotFound' handling. Found that previous implemenation was
buggy - threw a 500 as 'models.Integration.NotFoundError' that was removed
in previous commit didn't catch a needed error.
