Fix Wrapping Nested Tree Groups During Upload

[fm3]

Fixes a bug where nested tree groups were messed up during NML upload if “Create a new tree group for this file.“ is selected.

The group structure is a list of groups which can each have a list of children. When determining the largest free id, these children have to be taken into account as well.

URL of deployed dev instance (used for testing):

• https://fixwrapnestedgroups.webknossos.xyz

Steps to test:

• Upload an NML with nested groups, or one with un-connected trees within groups, like the one posted here https://scm.slack.com/archives/C5AKLAV0B/p1623331946023900?thread_ts=1623331839.023800&cid=C5AKLAV0B
• should upload fine, have stable group hierarchy
• should then download fine and re-upload fine.

Issues:

• fixes Importing NML with nested groups messes up hierarchy when using "Create a new tree group for this file" #5495

---

• Updated (unreleased) changelog
• Ready for review

[youri-k]

LGTM