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.
The hope here is that by moving all the schema files into synapse/storage/schema, it gets a bit easier for newcomers to navigate.
It certainly got easier for me to write a helpful README. There's more to do on that front, but I'll follow up with other PRs for that.
