Move testcases to yaml conversion and add filename to submissions

[the4t4]

Instead of storing the yaml format of testcases in the database, we now store the stringified json format as is given in the request. The conversion happens only for the grading script.

Also, now we save the filename of submissions in the grades table of our database. The filename is returned in the tasks submission get endpoint alongside the actual student's submission when the submission parameter is set to true.

Closes #82 #83

[Mohido]

Maybe the indentation of the output configuration needs a bit of tweaking. Otherwise, the code seems fine @the4t4