Discounts UI

#46 

Pippin's done an awesome job overhauling the Coupons UI in PR #72, which has been reviewed merged. There are just a few more things that need to be done to make it completed.

- [x] Remove use-x-times and free-shipping (these are legacy artifacts)
- [x] Use the plus/minus buttons to add /remove conditions
- [x] The fields that are created when "Add New Condition" is clicked are now translatable.
- [x] The status icons should be replaced with actual status text (probably with corresponding color) such as "Active" or "Inactive" or "Expired" etc.
- [x] Datepickers should be styled properly.
- [x] Deprecate functions that are not used any more.