Add a directive for file upload field in questionnaires

[markkuriekkinen]

A+/MOOC-Grader questionnaires already support file upload fields in questionnaires. MOOC-Grader recognizes file fields from the exercise YAML configurations. However, there is no RST directive in a-plus-rst-tools for adding file fields in questionnaires. The directive would be used inside questionnaires like the freetext and pick-one/any directives.

File upload fields in questionnaires would be particularly useful in manually graded exercises.