Create detached giftcards.

This reatains all original functionality but changes in the case when
both a variant and a line item are not linked. Essentially, this allows
the creation of gift cards without being attached to products.
Use a datepicker for expiration date
Allow the specification of any numeric value as a gc amount.

Replaces the select with a text field. By default, admin created gift
cards aren't bound to products.
Add a link to gift cards in the backend.

Currently there was no way to navigate to the url and it had to be
entered manually
Merge pull request #1 from Senjai/2-1-lav

Create detached giftcards.
Merge remote-tracking branch 'upstream/2-1-lav' into 2-1-lav