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 Still need to replace outdated screenshots; waiting for finalized UI.
Enrolled person using an external URL does indeed have to log in, but I think that would be a normal expectation to access course materials. Per our conversation I am going to leave the text as is.