Instead of being configurable, made xhr_paging always enabled.

This option made no sense to be turned off because it makes for a better
user experience to prevent a full page reload when paging.
Cleaned up preview windows after we've tested their content.
Now requiring only the jquery-ui JS that we use.
Merge pull request #2733 from refinery/enable-xhr-paging-by-default

Instead of being configurable, made xhr_paging always enabled.
Rectify changelog conflict.
Merge pull request #2736 from refinery/preview-functionality-cleanup

Preview functionality cleanup
Implemented support for turbolinks, when it's present.

To enable it, just place the following in your Gemfile:
gem 'jquery-turbolinks'
gem 'turbolinks'