deploy: move authelia's three host paths out of swarm.authelia

`usersFile`, `hostClientSecretDir` and `hostUsersFile` are filesystem
paths that only exist on the machine running the `swarm-authelia`
container. A hive that does not run it has nothing at any of them, so
they fail the swarm-wide test the namespace's header states.

`usersFile` is a path *inside* the container and still belongs on the
deploy side: a path's scope is the scope of the filesystem it names, and
that filesystem is this host's container root.

The two `host*` options are `readOnly` and derived, so nothing can set
them through the rename shims — those serve readers. The shims are still
required: `mkRenamedOptionModule` is what keeps an out-of-tree module
reading the old path resolving at all.

Seven reads went through an alias rather than a full path
(`autheliaCfg.hostClientSecretDir` in five modules, `.hostUsersFile` in a
sixth, `swarmCfg.authelia.hostClientSecretDir` in a seventh). Every one
of those files already binds `deployCfg`, so the repoint needed no new
alias. Four more sites were prose, including one in `docs/`, which no
grep restricted to `nix/` would have found.

nix/host-modules/hive-matrix.nix

@@ -799,7 +799,7 @@ in
       script = ''
         set -euo pipefail
 
-        src=${lib.escapeShellArg "${autheliaCfg.hostClientSecretDir}/${cfg.sso.clientId}.secret"}
+        src=${lib.escapeShellArg "${deployCfg.authelia.hostClientSecretDir}/${cfg.sso.clientId}.secret"}
         dst=${lib.escapeShellArg "/var/lib/nixos-containers/hive-matrix${toString deployCfg.matrix.sso.clientSecretFile}"}
 
         # authelia's container is up, but its first-boot generator may