After merge, remove segment item of id that doesn't exist anymore

[philippotto]

• When merging two segments, removes the segment from the segment list.
• The active segment/item now always survives the merge operation (wasn't the case when merging agglomerate-skeleton or skeletons with a shortcut).

URL of deployed dev instance (used for testing):

• https://removesegmentaftermerge.webknossos.xyz

Steps to test:

Part 1:

• create a new annotation for a dataset that has an agglomerate mapping (e.g., test-agglomerate)
• activate proofreading mode
• click on two segments that should be merged so that they have an entry each in the segments tab
• merge them with the context menu
• the segment id that doesn't exist anymore in the data should also not exist anymore in the segments tab
• repeat the above by loading agglomerate skeletons and merging them (make one node active and rightclick another node -> create edge)
  Part 2:
• go into skeleton mode
• create two trees
• merge them with the ctrl+shift+alt shortcut --> active tree should survive
• merge them with the context menu --> active tree should survive

Issues:

• fixes [Proofreading] Remove segment from list after merge #7315

---

(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

[daniel-wer]

Nice one, LGTM 👍