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/swarm-nats.nix

@@ -57,7 +57,7 @@ let
   responderConfigured =
     deployCfg.nats.autoGenerateCallout
     || (deployCfg.nats.calloutUserSeedFile != "" && deployCfg.nats.calloutIssuerSeedFile != "");
-  clientSecretSource = "${autheliaCfg.hostClientSecretDir}/${cfg.clientId}.secret";
+  clientSecretSource = "${deployCfg.authelia.hostClientSecretDir}/${cfg.clientId}.secret";
   introspectionUrl = "${toString autheliaUrl}/api/oidc/introspection";
 
   # Where the responder's seeds actually come from. One name for two