Enable enforcement of a specific format of submissions

[Evgeny Kotelnikov]

Most courses and labs specify the format of the submission -- e.g. "a solution is two files Lab1.hs and Wrapper.hs, containing...". It could be useful to enforce the assign the format of submissions to a lab and enforce the uploaded files to match it.

[Evgeny Kotelnikov]

Labs are now assigned with lists of file extensions they take.

[Gregoire Detrez]

The file extension constraint should be enough for simple situation (like allowing only pdfs) and automated checks can be used for arbitrary complex cases (number of files, name, content...)