Moves the matrix sub-domain vhost out of the gateway's vhosts.nix, its
`address=` rule out of dnsmasq.nix, and the Accept-header
`$matrix_spa_target` map out of the gateway's appendHttpConfig — all
three into hive-matrix.nix.
The map is the one that had no business being where it was: it exists
solely for the SPA fallback in the vhost's `/` location, and
`appendHttpConfig` is a `lines` option, so a module can contribute to
it without the gateway assembling it.
The `.well-known/matrix/*` delegation deliberately stays on the hive's
own vhost. The spec requires it at the SERVER NAME, which is the hive
domain: that is the hive answering "where is my homeserver", not the
homeserver answering for itself. Moving it would have been the obvious
symmetric thing and it would have been wrong.
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