a user can view their gift cards

Add new gift_card action to the user controller decorator. View shows
all the user's active gift cards, sorted by date to expire. Passing
?show_all=true will inclue the expired gift cards.

Change-Id: Ifd19861ac58395855ece6684a1852b3b818bf06c
Add a link in users#show to view giftcards

Change-Id: I9d7e530e171dbf19c719ef98eb552112654d646e