Add support for export functionality

[Heartfirey]

Here are some new features I recommended about export functionality

1.Added support for contest data export.

As we all know, every contest will generates a lot of data, for example: List of participants and final list of competition. So the function of export data is is particularly important.
At the same time, I suggest that the original format be retained in the exported document, such as green when passing and red when submission fails.

2.Added support for homework data export

I recommended NOJ added homework data export, and the filtering function should be set during export to limit the export scope of homework, users etc.
Homework can be a fantastic new function for NOJ, this function will donate a lot of help to daily training. However, as we all know, the new functions still need to be gradually improved. So I think this function can be regarded as one of the key points of the next version.

I highly recommended NOJ to added export function, and add multi-way support for it at the same time(in excel, csv, html etc.). Thank you very much!