Bunch of fixes for code submission backend

[ali-ramadhan]

Bunch of fixes:

1. The "submit code" button was relying on the value of the language dropdown to determine the submission's language. So lots of people were probably getting weird errors when trying to submit code files. The language is now determined from the file extension. So this actually resolves Submitting code via upload button doesn't work #57.
2. I was messing up the file paths for code submissions and they were getting saved in two places with different file paths. This is now fixed, and each submission just saves one file in one place. I won't delete or reorganize previous submissions since this will require rewriting database entries which doesn't seem worth it.

Resolves #57
Resolves #104