We've previously changed a bunch of lists in `homeserver.yaml.j2`
to be serialized using `|to_nice_yaml`, as that generates a more
readable list in YAML.

`matrix_synapse_federation_domain_whitelist`, however, couldn't have
been changed to that, as it can potentially be an empty list.

We may be able to differentiate between empty and non-empty now
and serialize it accordingly (favoring `|to_nice_yaml` if non-empty),
but it's not important enough to be justified. Thus, always
serializing with `|to_json`.

Fixes #78 (Github issue)