Add release events to release serialization.
Add duration serialization for releases.
Add medium formats to release serialization.
Only include unique aliases in Role::Aliases. Don't worry about sorting, either.
Add labels and catalog numbers to release serialization.
Only include the @context at the top level.
Merge branch 'json-ld-releases-1' into beta

* json-ld-releases-1:
  Only include the @context at the top level.
  Add labels and catalog numbers to release serialization.
  Only include unique aliases in Role::Aliases. Don't worry about sorting, either.
  Add medium formats to release serialization.
  Add duration serialization for releases.
  Add release events to release serialization.