Tournament-specific styling

[JobDoesburg]

In order to make this product really versatile, I propose to implement tournament-specific styling (after #81 has been implemented).

Don't worry, this is easier than you might think.

Via the admin for every tournament, you can select a theme color and perhaps a font family (and maybe upload a favicon?). These are simply added to the API's tournament serializer and the frontend just applies them. That's it.

Additionally, we could implement a field on tournaments with custom CSS, that we also simply just load. This is exactly done in this way in CTFd and it works perfect