It's difficult to submit in "variant languages"

[Matistjati]

In one large contest Kattis is currently holding, Python3 with NumPy is allowed, but not Python 3, which was pretty confusing for one team, as they didn't manage to figure out how to submit using the CLI (just running submit.py on a Python file will get you an "Invalid language" response). Not sure if there are any good solutions, but I think it should at least be documented.

Perhaps we should at least make some note on the language names of the "variant languages" in the source code, and a sample command on how to submit in them. It would also probably be helpful to distinguish between "Invalid language" and "Language not allowed in this contest".

[JoelNiemela]

I think the best way to solve this issue would be to fetch the list of available languages in a contest before submitting. Then the submit-client can use that list to make a better language guess.

To do this, we also need to improve how language guesses are handled — currently, a single file-ending can only "belong" to one language, which would need to change. If we do these improvements to the language-guess code, I think it would be worth it to move away from storing the _LANGUAGE_GUESS dictionary in the submit-client code itself, and rather fetch it from the Kattis server. This way, we won't have to make an update to the submit-client every time a new language is added (or modified).