Draft: Let label references be derived for a batch of events

Closed: this two-line change was folded into !255091 (merged), which now writes the reference at the entry points directly. Nothing here is lost.

Makes ResourceLabelEvent#label_reference return an empty string when the label has been deleted, instead of raising. MR 3 needs this: its write-time callback runs on every create, including rows whose label is already gone.

Part of the stack for #628135 (INC-12449). Merge in order:

1. Render group level work item label notes against the group
2. Let label references be derived for a batch of events <-- this MR
3. Store the label event reference when the event is written
4. Delete label events when their label is deleted
5. Link a project label in a group context to its project
6. Backfill the reference on existing label events
7. Stop writing the label event reference on a read request

Targets 628135-01-work-item-note-parent, not master.

Testing

A new #label_reference block in resource_label_event_spec covers the bare and qualified forms for project and group labels, plus the empty string for a deleted label.

Related to #628135