Create transaction based on user instead of card

[stickyPiston]

Currently to create a new transaction, a uuid of a card should be sent to /api/transaction. However, in the create_transaction route the card is only used to find the corresponding user. Therefore i'd like to propose to change the route's uuid parameter to work based off user id instead of card id. This does mean that magnificent-sloth needs an update too, but after some investigation, that doesn't seem to need too much changing.