Adds highscore feature to multiplayer

[ComputasAlex]

Multiplayer will be an own game mode in terms of highscore. This is backed by the fact that configuration is different for multiplayer than singleplayer. In the db this is reflected in an own identifier for the multiplayer mode in the difficulty_id column on Scores. Added DifficultyId enum to utilities to have this under control.

Some of the code for high score fetching from db is very similar to singleplayer backend and this highlights the need to refactor the whole architecture, but this is a bigger scope than for the last 3 weeks of the summer project.

New websocket endpoints:

• postScore:
  • Every multiplayer game now also posts scores to db with the Multiplayer DifficultyId 4.
• viewHighscore
  • Essentially the same as in singleplayer backend