Validator does not test for label in v3 manifests

[benwbrum]

The spec states that A Manifest must have the label property with at least one entry. Currently the validator does not test for the presence of the label element in 3.0 manifests.

To reproduce:

• Inspect https://iiif.io/api/cookbook/recipe/0014-accompanyingcanvas/manifest.json and verify that no label element exists for the manifest itself.
• Run the online validator with https://iiif.io/api/cookbook/recipe/0014-accompanyingcanvas/manifest.json
• The manifest should be invalid, but no errors are detected.

[glenrobson]

I believe this is fixed in: 36b3e9d

and I've tested it locally now and it does pick up a missing label.