Allow the mautrix whatsapp relaybot to be enable with a variable

This allows a user to enable the relaybot by setting a variable in
`vars.yml` in the same way that the mautrix signal relaybot is
configured.

roles/custom/matrix-bridge-mautrix-whatsapp/templates/config.yaml.j2

@@ -396,9 +396,9 @@ bridge:
     relay:
         # Whether relay mode should be allowed. If allowed, `!wa set-relay` can be used to turn any
         # authenticated user into a relaybot for that chat.
-        enabled: false
+        enabled: {{ matrix_mautrix_whatsapp_relaybot_enabled }}
         # Should only admins be allowed to set themselves as relay users?
-        admin_only: true
+        admin_only: {{ matrix_mautrix_whatsapp_relaybot_admin_only }}
         # The formats to use when sending messages to WhatsApp via the relaybot.
         message_formats:
             m.text: "<b>{{ '{{ .Sender.Displayname }}' }}</b>: {{ '{{ .Message }}' }}"