Use asset_path helper in select2 styles with urls.

This also moves the images for select2 into vendor/assets/images and configured
the app to precompile assets in vendor/assets/images.

:fork_and_knife: :sweat_smile:

Chatted with @bcobb to arrive at this solution. I think it is what we want to wrap up https://trello.com/c/UN4xLTmj