Fixes and updates to match master branch with Spree 2.3.0.beta

Please review my commits

-Remove 'size' from field inputs since 'fullwidth' class is used
The 'size' option isn't really needed since a previous commit added the 'fullwidth' class

-Combine views and update to Spree usage along with drag sorting
General updates that match how other spree views function

-Remove presence validation covered by inclusion
No need to tell the user twice that their update fails

-Fix 'select2' select boxes when adding rows and change to append
The select2 combo-box that Spree uses (maybe not used in 'test' ENV?) is now works with field rows
I changed prepend to append for adding fields since it makes more sense to add to the end of the table

-Update asset paths to match Spree
Folder rearrangement with generator fix so rails g spree_volume_pricing:install works correctly

-Fix @variant.volume_prices.build
Issue when using Rails 4.1 (placement of super in def?)