Replace _data/public submodule with import

The _data/import-public.rb script does the heavy lifting. I imagine scripts for 18F/18f.gsa.gov and 18F/Dashboard being similar, though they won't do the snippet scrubbing at all.

@afeld @konklone @gboone @jackiekazil
We'll need some variant of this for dashboard, the homepage, etc., right? Not a blocker, but we should think about how to share the logic.