Add ECSC task parser

[JJ-8]

With the upcoming ECSC competition, it is useful to have a special ECSC task parser since some teams will use CTFNote as their note management software, and importing all the tasks yourself is just painful.

This parser parses the JSON output of the /stats.json endpoint of the ECSC gameboard. The output doesn't contain actual categories, so we use the 'contract' name as the category.

Example JSON output of /stats.json to test the parser:

[
    {
        "task": "First task of first contract",
        "contract": "First contract",
        "cash": 0,
        "solved_by": [
            "test"
        ],
        "average_time": "35 seconds"
    },
    {
        "task": "Second task of first contract",
        "contract": "First contract",
        "cash": 0,
        "solved_by": [],
        "average_time": null
    },
    {
        "task": "first task of second contract",
        "contract": "second contract",
        "cash": 10,
        "solved_by": [],
        "average_time": null
    },
    {
        "task": "second task of second contract",
        "contract": "second contract",
        "cash": 10,
        "solved_by": [],
        "average_time": null
    }
]

[XeR]

Good work !
I'll try to get some important person to test it on ENISA's beta environment. I'll merge it when I have their feedback