Trying to:
- stay closer to naming in Synapse (autojoin -> auto_join)
- not create new variable namespaces (`matrix_homeserver_`),
when existing ones (`matrix_synapse_`) are more suitable
- allow `null` (`~`) values for `matrix_riot_web_welcome_user_id`
- render things like `auto_join_rooms` in `homeserver.yaml` more prettily
- fix breakage in `config.json` where `matrix_riot_web_roomdir_servers`
was rendered as YAML and not as JSON
- simplify code (especially in riot-web's `config.json`), which used
`if` statements that could have been omitted
- avoid changing comments in `homeserver.yaml` which are not ours,
so that we can keep closer to the configuration file generated by upstream
bc15db3316