Allow to remove fallback layer

[grittaweisheit]

This PR makes it possible to remove the fallback layer in a volume tracing. To do this a button in the layer settings header was added.

URL of deployed dev instance (used for testing):

• https://___.webknossos.xyz

Steps to test:

• start a volume or hybrid tracing with existing segmentation layer
• go to the dataset settings
• next to the segmentation layer's settings header should be a minus button
• on click this should open a confirm pop up
• when selecting ok the layers segmentation should be removed and the layer should be reloaded
• the minus button should disappear
• when doing a volume/hybrid tracing with a task the button should not be there

Issues:

• fixes Allow to remove fallback layer from a volume tracing #4741

---

• Updated (unreleased) changelog
• Updated (unreleased) migration guide if applicable
• Updated documentation if applicable
• Adapted wk-connect if datastore API changes
• Needs datastore update after deployment
• Ready for review

[philippotto]

Awesome! This is a really cool change which will allow us to remove some cognitive load from the users' minds 🎉

I'd suggest a few subtle changes, though:

• <Icon type="stop" /> fits a bit better in my opinion
• Change the wording to "Unlink dataset's original segmentation layer" ("Remove Layer" could be confusing since the actual volume layer will still be there) in the button's tooltip
• Add a more detailed explanation to the modal. I had a discussion with Tom and he suggested:

Are you sure you want to unlink the dataset's original segmentation layer from this tracing?

This dataset already contains a segmentation layer provided by its author. If you do not wish to base your work on this original segmentation, you can unlink it by confirming this dialog.
Note, that this action cannot be undone. Also note, if you already started with your annotation work based on the original segmentation layer, some small chunks of the segmentation might have already been merged into your annotation for technical reasons.

[philippotto]

Awesome! I left some final comment about tweaking the modal's appearance a bit. After this is done, this should be ready to be merged :)