move CartItem initialization code out of Cart#import_cart_items
refactor Cart#process_approvals_without_approval_group
move CSV methods into CartItem
refactor Cart#create_comments_csv
refactor TimeHelper
abstract setting of multiple properties
create service object for exporting CSVs
make smaller methods in Cart
split up exporter
Merge pull request #113 from 18F/refactor-cart

[#82417004] refactor Cart