Fix Mapping column handler
Fix Mapping column handler

The find_related function also works without an id and matches any
object of the given type. That prevents a mapping to be created if a
similar mapping already exists between two objects of the correct type.

AleksNeStu/ggrc-core,AleksNeStu/ggrc-core,AleksNeStu/ggrc-core,AleksNeStu/ggrc-core