Small bugfix related to #55

* Add default template file for homepage HTML
* Add default riot-web config options for homepage

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

This playbook does not set up guest access in Synapse anyway,
so until the need comes (or someone asks for it), guest access
is removed from riot-web's UI too.

As for supporting custom URLs, this is also not something
that seems like it'd be useful to most deployments.