Create endpoint mocks for the organisation library

[TomasEng]

In order to be able to work on the frontend and the backend on the organisation library in parallell, we need a solution for mocking the endpoints when they are called from the frontend.

See #14482 and #14503 for lists of endpoints that we need.

[wrt95]

Endpoints to implement mock for in frontend:

• Endpoint for fetching code lists (GET)
• Endpoint for creating a code list (POST)
• Endpoint for mutating a code list (PUT)
• Endpoint for deleting a code list (DELETE)
• Endpoint for fetching the text resources of a given language (GET)
• Endpoint for creating a list of text resources for a new language (POST)
• Endpoint for mutating a list of text resources for a given language (PUT)