Support non-ipynb submissions

[salikh]

Currently autograder accepts only one kind of submissions -- ipynb notebooks-- , and it finds the metadata inside of the ipynb file.

There are use cases for the autograder where the submission is not a notebook:
-- Submitting HDF5 (h5) model files created by Keras (important in courses related to Machine Learning)
-- Submitting stand-alone python scripts (important in Python courses to teach students to use stand-alone scripts)

Since the server requires metadata to match a submission to the assignment, the upload server should be extended to support taking the metadata in addition to the submission file upload.

There are a few options for inferring metadata:

• The upload form can take additional input parameters, which could be provided by a form or as a hidden form input at the client side.
• Upload file name could serve as a key in metadata lookup, where the metadata is stored on the server in advance
• Upload URLs can encode metadata key inside (i.e. the upload URL is different for each assignment)