Add target info to "Library.change!" pubsub message

[byewokko]

Summary

• LibraryService.subscribe is extended with optional target argument.
• Possible target values:
  • None (default) or "global" to watch global target changes,
  • "tenant" to watch changes in ANY tenant,
  • ("tenant", TENANT_ID) to watch changes in a specific tenant.
• The callback method must expect target argument, whose value is either "global" or ("tenant", TENANT_ID).
• Backward compatibility: When LibraryService.subscribe is called without target argument (or its value is None), the callback is called without target argument.

Other changes

• Fix: Superuser has access to any tenant.