* 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.

* Correct default values for mautrix whatsapp relaybot variables

* Add documentation for using the relaybot with mautrix whatsapp

* Adjust variable names to better reflect what they do

* Set default variables properly and use to_json in template