If uppercase is used, certain tools (like certbot) would cause trouble.
They would retrieve a certificate for the lowercased domain name,
but we'd try to use it from an uppercase-named directory, which will
fail.

Besides certbot, we may experience other trouble too.
(it hasn't been investigated how far the breakage goes).

To fix it all, we lowercase `host_specific_hostname_identity` by default,
which takes care of the general use-case (people only setting that
and relying on us to build the other domain names - `hostname_matrix`
and `hostname_riot`).

For others, who decide to override these other variables directly
(and who may work around us and introduce uppercase there directly),
we also have the sanity-check tool warn if uppercase is detected
in any of the final domains.