contents.js wrapper for contents service

This is PR #6671, rebased because it's a blocker for the text editor components, and @fperez asked me to make that high priority. So far, I've just rebased, I intend to test it and fix things I see.
Is this intended for the checks used to serve local files using ContentsManager? It's not clear how that can work at all if ContentsManager is reimplemented on the client.