During the migration the automated script to update the copyright
headers accidentally got rid of some of the existing copyright lines.
Reinstate them.
If configured an OIDC IdP can log a user's session out of
Synapse when they log out of the identity provider.
The IdP sends a request directly to Synapse (and must be
configured with an endpoint) when a user logs out.
This implements a fake OIDC server, which intercepts calls to the HTTP client.
Improves accuracy of tests by covering more internal methods.
One particular example was the ID token validation, which previously mocked.
This uncovered an incorrect dependency: Synapse actually requires at least
authlib 0.15.1, not 0.14.0.
