#534 Added gift card background image
#534 Added new inputs to pricing form

Not yet super pretty
#534 Added gift card partial and prepared styles
#534 Some minor formatting changes
#534 Removed checkbox again, because it's actually pointless
#534 Layout fix for time pass description
#534 Fixed styling of select2
#534 Removed obsolete landing page URL input
#534. Added shortcodes functional and gift codes view.
#534. Merge with last changes.
#534. Added gift code generation and access functionality.
#534 Renamed $code -> $voucher for consistency
#534 Renamed cookie for clarity; highlighted undocumented spots
#534 Renamed has_purchased_gift -> has_purchased_gift_card; highlighted undocumented functions
#534 Improved comment
#534 Renamed $gifts -> $gift_cards
#534 Why not use the existing redeem voucher markup?
#534 whitespace
#534 More comments
#534 Renames: pass -> time pass; $is_gifts -> $is_gift