During the migration the automated script to update the copyright
headers accidentally got rid of some of the existing copyright lines.
Reinstate them.
Cleans-up the schema delta files:
* Removes no-op functions.
* Adds missing type hints to function parameters.
* Fixes any issues with type hints.
This also renames one (very old) schema delta to avoid a conflict
that mypy complains about.
These columns were added back in Synapse 1.52, and have been populated for new
events since then. It's now (beyond) time to back-populate them for existing
events.