Mweb 965

Before we switch to another project we want to clean up. One of the worst scored by Code Climate files were our CRUD files. In this pull request you can find changes which clean up this part of the service. The clean up includes:
* moving everything that looks like configuration to configuration files,
* moving helper functions to utils modules,
* extracting common logic for all cruds to common utils module,
* flattering promises' calls,
* unit tests.

We also made small changes in HTTP code the service returns -- it'll fix our Python automation API tests.

This pull request changes' are connected to `Wikia/app` changes as well:
* https://github.com/Wikia/app/pull/4899