Fixes bug with removing ListingChanges' annotations

- Removes accepted_nested_attributes' reject_if;
- Adds code to clear annotations on the before_save callback for listing_changes;
- Adds spec to test behaviour;
- Renames annotations, listing_changes, and listing_distributions attribute from source_id to original_id to avoid confusion with the Source trade code type;

Fixes Pivotal Tracker story: https://www.pivotaltracker.com/n/projects/623923/stories/73782068