Branch session in the wrong state

The branch session cannot do this now: only its owner edits it, only open branches take edits and submits, and merged or discarded branches are final.

#Common causes

• Editing a branch after branch.submit, or someone else's branch.
• Merging or discarding a branch twice.

#Fixes

• Open a new branch session with branch.open for further work.

• Code: ARA2506

• Constant: collab::BRANCH_STATE

• Reported by: ara-collab