use heroku_rails_deflate gem to gzip rails content and static assets

The [heroku_rails_deflate](https://github.com/mattolson/heroku_rails_deflate) gem [inserts Rack::Deflater](https://github.com/mattolson/heroku_rails_deflate/blob/master/lib/heroku_rails_deflate/railtie.rb) at the appropriate stage in the Rack middleware stack, and also contains code to [serve static gzip files](https://github.com/mattolson/heroku_rails_deflate/blob/master/lib/heroku_rails_deflate/serve_zipped_assets.rb) that are already being generated by `assets:precompile` rake task. This should enable full gzip compression of our javascript/css/html content, reducing page load times and bandwidth costs.

Please review @laurelfan @geoffrey-elliott