migrate existing revisions metadata extra_headers to actual extra_headers field

New revisions now write their extra_headers in a dedicated fields. Older ones were writing it within the metadata > extra_headers one. Those need migrations.

Following the run of D3820 and the run of this migration. We'll then be able to drop the 'metadata' field from revision.

---

Migrated from T2564 (view on Phabricator)