validate Approval#status
simplofy signature for CommunicartMailer.approval_reply_received_email
remove usage of arbitrary param hash for approval email
remove use of @cart_template
move cart template logic into decorator
remove use of ternary operator

...for better readability and test coverage reporting
unify approval logic
Merge remote-tracking branch 'origin/master' into approval-refactor

Conflicts:
	app/models/cart.rb
Merge pull request #88 from 18F/approval-refactor

Approval refactoring