Function: get_back_face

--Usage--
> get_back_face(input: card)

Inspects a given [[type:card]]'s links to find one of type "Back Face", and returns the linked card.
Returns nil if no link of type "Back Face", or no card was found.

--Parameters--
! Parameter	Type				Description
| @input@	[[type:card]]		The card whose links we'll inspect.
| @set@		[[type:set]]		The set in which to look. This can be omited since 'set' is a predefined variable.
