[frontend/tasks] Cap the student shown file upload limit to the server's limit.

[anthonygego]

Is your feature request related to a problem? Please describe. Teachers can set the maximum size for their file upload problems, but they can virtually sets a larger size than the one accepted by the web server. Therefore the students don't understand why the file is not uploaded.

Describe the solution you'd like A limit can be specified in the webapp config so that the subproblem class can access the information.

Describe alternatives you've considered Maybe we can propagate this from the webserver to the wsgi options, but would it be convenient ?