vhosts: Only enable vhost config if it exists.

We're going to create a module-list.nix file like in the official NixOS
repository, so we don't want to have unnecessary services hanging around
on all nodes.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Add all modules to a new module-list.nix file.

This now makes it unnecessary to explicitly add all those modules to the
corresponding imports attribute and also centralizes the handling and
inclusion of those modules.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
services/mongooseim: Add missing descriptions.

Those were only used for internal options, but necessary nonetheless for
building the manual.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
release: Add manual listing our specific options.

This manual collects all options from our module-list.nix and generates
a HTML manual that formats them nicely.

I'm not an expert if it comes to DocBook, so this is heavily inspired by
the way it is done in the official NixOS manual.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>