Using Rails asset pipeline; moved assets to app/assets; updating lots of paths

The paths of images, JS, and CSS will all change to /assets (e.g. /assets/application.js). In production, since nginx is serving the assets, it is necessary to precompile all assets into a single folder. This can be done with by running `rake assets:precompile RAILS_ENV=production` after updating the code

Also, nginx.conf will have to be updated to reference this new location for static content
This setting has to do with precompiling, and [telling the precompiler how old something needs to be to delete it from the server](https://github.com/ndbroadbent/turbo-sprockets-rails3#removing-expired-assets). I would recommend we set the nginx expiration to be very high like 1 year, and keep `config.assets.expire_after` around 2-8 weeks so we don't keep too many older versions of assets on the app servers