Add a file to themes with sample data

[jacobobryant]

trafficstars

If a theme has an input.edn file (i.e. committed to the git repo, not the input.edn file that platypub generates), treat it as sample data. When you create a new site with that theme, populate the db with the sample data. Sample data files will also be useful for doing theme development with ./task dev in the theme folder, but that already works.