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 Still need to replace outdated screenshots; waiting for finalized UI.
I know you're just updating this, but I'd frame this in terms of the consequences: To refer to the file from within the course, you must use the Studio URL. If you use the External URL, this bad thing happens.