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.
Why isn't this `"Using contents: %s" % self.settings.g...` or `"Using contents: {0}".format(self.settings.g...`?