Now that discs are merged into one release, we can't link catalogue numbers to the discs they belong to.
See http://test.musicbrainz.org/release/70be6a6d-d3c4-4489-a6ac-249cb8aca950 for example
Extend dynamic attributes to all entities
Add "Primary" flag to catalog numbers
Allow barcodes on mediums
Add “primary” flag to release labels
Link catalogue numbers to release only (release-catno duet instead of label-release-catno triplet)