Editable pages

This implements the "Edit this page" mechanism for content under pages/. Inspired by https://github.com/blacktm/standard-jekyll-template.
It means it doesn't require a code change to a plugin when adapting the Hub to a different team. Granted, adapting the Hub to a different team may require plugin changes, but I think it's best to minimize those, since that would seem to increase friction where adoption's concerned.

Or are you suggesting a different alternative? Like baking it into a layout, and using the layout to set the defaults for pages/ and pages/private/?