waiverdb: Fix redirect_uri for new flask-oidc versions
    
    For details, see the changelog for flask-oidc:
    https://github.com/fedora-infra/flask-oidc/blob/develop/docs/changelog.md
    
    The entry for 2.0.0 says:
    
    > The callback route (aka "redirect URL") is not configurable with
    > `OIDC_CALLBACK_ROUTE` anymore. It is always `/authorize`,
    
    The option `OIDC_CALLBACK_ROUTE` is available again in version 2.0.3,
    but we can override the whole redirect URL with
    `OIDC_OVERWRITE_REDIRECT_URI` (since 2.1.0).