default value for webhook URL

It would be really great if there was an option to set a configuration variable to set the `Notify Webhooks` URL instead of having to manually populate the same string every time we add a Web URL Resource.

Along with other configuration variables as outlined in the docs [here](https://docs.geohealthcheck.org/en/latest/config.html#configuration-parameters), perhaps something like `GHC_WEBHOOK_URL` to auto-populate the value as a default string would be excellent.