Make active tree node round instead of shifting the hue value

[philippotto]

URL of deployed dev instance (used for testing):

• https://roundactivenode.webknossos.xyz

Steps to test:

• open a skeleton tracing and change the active node (e.g., create new nodes)
• the active node should be round and should have the same color as the other nodes of the same tree

Issues:

• fixes Improve highlighting of active node  #3575

---

• Updated changelog
• Ready for review

[daniel-wer]

LGTM, although the active node is pretty hard to distinguish now imo, but I don't have a better idea :)