Files & Uploads filtering (DOC-1316)

@martynjames @marcotuts @mhoeber @srpearce @lamagnifica please review
This is an initial draft. Still need to replace outdated screenshots; waiting for finalized UI.
maybe an intro statement about why you care about these URLs? like, To include a link to the files that you upload in your course, you obtain the URL of the file and then add it to the course component blah blah