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
The other repos will need something similar up to line 41, before the snippets import starts.