Difficulty and Alpha changeable

This was a very small update which took me way too long

• It is possible to change the alpha of the chess board to either remove it completely or make it fully visible
• It is possible to choose your own difficulty for the skirmish matches
• There was a bug with the seasons. Now everything works fine again
• Discord link is implemented in credits plus some text changes

That's it,...
not much,...
but nonetheless I'm proud to have finished this...
currently I lack the motivation to code beside my day job, so it was good to motivate myself and get this finished.

Feedback is, as always, highly appreciated.