Fixes !27: Validate FormProcessorAction mappings on save.

Issue !27 (merged) is actually two separate bugs, but they're closely related - they both occur because when we save a FormProcessorAction, we don't check the validity of the mapping. If we delete a custom field referenced in the mapping, we must also delete it from the mapping array. Likewise, if we delete an input that is referenced in the mapping, we must remove it from the mapping. I've introduced a FormProcessorAction mapping validation method, which can be expanded if other invalid mapping permutations arise.