When merging annotations, deduplicate bounding boxes

[fm3]

When we introduced user bounding boxes in https://github.com/scalableminds/webknossos/pull/4536 we already talked about deduplicating the bounding boxes during annotation merge (both backend and frontend). We discarded the attempts at the time, because the first implementations didn’t perform as expeted and we didn’t want to block the PR. Now users have reported that the bbox buildup is annoying. We should look into that again.

Should be done in:

• [ ] back-end
• [ ] front-end