Extract API to a separate app

Currently the API is included in the "championship" app, which roughly includes everything running on unityleague.ch, so creating the new events, importing results, and so on.

However, I would like in the future to implement some API for the invoicing, or possibly for future features such as decklists, and I think it does make sense to have it in a separate app that imports models from championship and invoicing and so on, as needed.

First commit does not change any code, just move things around. Second commit actually delets duplicates URLs.