Uploading of files in Tutor Response Phase

Uploading of files in Tutor Response Phase can be uploaded into a public repo.
The url of the public repo can possibly be defined in `settings.json`.

This can probably solve the problem of invalid file url generated from Github.

Start by tracing file uploads in the Tutor Response Phase and reviewing settings.json for the proposed public repository URL. Clarify how uploads should be stored and how generated URLs are returned; the work is done when uploaded files use the configured public repository and produce valid URLs.

Written by the indexing model from the issue text.