Lower z-index for "Remove layer" trashcans in remote dataset import

[dieknolle3333]

URL of deployed dev instance (used for testing):

• https://floatingtrashcansremotedsimport.webknossos.xyz/

Steps to test:

• Upload remote dataset and add another layer
• make viewport smaller so that you can scroll layers (see issue for GIF)
• make sure trashcans to remove layer dont float above the navbar

TODOs:

• see whether issue after import persists (again, floating trashcans) -> couldnt reproduce it
• look for alternative to z-index hacking (didnt really find anythings thats suitable https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_positioned_layout/Understanding_z-index/Stacking_context)

Issues:

• fixes Floating trash cans on remote dataset import #7303

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(Please delete unneeded items, merge only when none are left open)

• Updated changelog
• Updated migration guide if applicable
• Updated documentation if applicable
• Adapted wk-libs python client if relevant API parts change
• Removed dev-only changes like prints and application.conf edits
• Considered common edge cases
• Needs datastore update after deployment

[philippotto]

lgtm 👍