Add "extra" field to display all footnotes

Re-rolled patch against 3.1.0

Removed Patch created. No change. Uploaded same patch by mistake.

Going to merge this into 4x and created a follow-up here 📌 Follow-up to all footnotes available as a block to apply to modal plugin Active to apply it to the new modal version before the old one gets retired. Patches still needed while on 3x until 4x (and its upgrade path are more stable).

Automatically closed - issue fixed for 2 weeks with no activity.