Fix |to_nice_yaml breaking formatting

Well, actually 8cd9cde won't work, unless we put the
`|to_nice_yaml` thing on a new line.

We can, but that takes more lines and makes things look uglier.
Using `|to_json` seems good enough.

The whole file is parsed as YAML later on and merged with the
`_extension` variable before being dumped as YAML again in the end.

roles/matrix-synapse/templates/synapse/homeserver.yaml.j2

@@ -968,7 +968,7 @@ account_validity:
 #  - email
 #  - msisdn
 {% if matrix_synapse_registrations_require_3pid|length > 0 %}
-registrations_require_3pid: {{ matrix_synapse_registrations_require_3pid|to_nice_yaml }}
+registrations_require_3pid: {{ matrix_synapse_registrations_require_3pid|to_json }}
 {% endif %}
 
 # Explicitly disable asking for MSISDNs from the registration
@@ -987,7 +987,7 @@ registrations_require_3pid: {{ matrix_synapse_registrations_require_3pid|to_nice
 #  - medium: msisdn
 #    pattern: '\+44'
 {% if matrix_synapse_allowed_local_3pids|length > 0 %}
-allowed_local_3pids: {{ matrix_synapse_allowed_local_3pids|to_nice_yaml }}
+allowed_local_3pids: {{ matrix_synapse_allowed_local_3pids|to_json }}
 {% endif %}
 
 # Enable 3PIDs lookup requests to identity servers from this server.