Enable mappings in volume/hybrid tracings

[daniel-wer]

... that are based on an existing segmentation. For volume tracings with a completely new segmentation, it doesn't really make sense to apply mappings from the server.

URL of deployed dev instance (used for testing):

• https://___.webknossos.xyz

Steps to test:

• Open a new volume tracing based on the existing segmentation! on a dataset that has mappings.
  • You should be able to use the mappings UI to list the available mappings and select specific mappings.
  • You should be able to programatically add a new mapping, e.g.: using webknossos.apiReady(3).then(async (api) => {api.data.setMapping(api.data.getVolumeTracingLayerName(), {7353: 1337})}) (replace first id with one you can see in the view)
• Selecting mappings in a skeleton tracing or when viewing a dataset should still work

Issues:

• fixes Enable segmentation mappings in tracing modes #3863

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• Updated changelog
• Ready for review

[philippotto]

Perfect PR 👍 Works flawlessly, too 🎉