CMS snippets

This PR is part of the effort to make 3000 Acres more configurable without code changes.

Main features:

1. The CMS is set up automatically when you run "rake db:setup", rather than needing a lot of manual fiddling
2. "About", "Media" and "Resources" pages (all blank) are set up by default when you create the site
3. A bunch of snippets are created in the CMS, which contain most of the content of the homepage and a few other bits of the site.  You can now edit these to change the wording etc, just by editing them in the CMS, without requiring the dev team to make the changes in the code.