Add several helper methods returning SWHIDs

This adds several helper methods returning SWHIDs to model objects,
namely:

- SkippedContent.swhid()
- DirectoryEntry.swhid()
- SnapshotBranch.swhid()
- Release.target_swhid()
- Revision.directory_swhid() and Release.parent_swhids()
- OriginVisitStatus.origin_swhid() and
  OriginVisitStatus.snapshot_swhid()