Fix a couple type annotations in the `RootConfig`/`Config`. Discovered
while cribbing this code for another project.
It's really sucks that `mypy` type checking doesn't catch this. I assume
this is because we also have a `synapse/config/_base.pyi` that overrides
all of this. Still unclear to me why the `Iterable[str]` vs
`StrSequence` issue wasn't caught as that's what `ConfigError` expects.
During the migration the automated script to update the copyright
headers accidentally got rid of some of the existing copyright lines.
Reinstate them.
* raise a ConfigError on an invalid app_service_config_files
* changelog
* Move config check to read_config
* Add test
* Ensure list also contains strings
