Point from the dashboard to Stalwart's own administration

A line under the cards says where the rest is: detailed metrics, the
delivery queue, logs and server settings are in Stalwart's own
administration. It links there when the operator sets STALWART_ADMIN_URL,
and stays plain text otherwise, because STALWART_URL is how this server
reaches Stalwart and is often an address no browser can open.

Several servers: a servers file entry may now be an object,
{"url": ..., "adminUrl": ...}, and a session routed to that server gets its
adminUrl. A routed domain without one gets no link rather than the default
server's, for the same reason routing never falls back. The URL is sent
only to a session that may administer.

The shipped example file stopped the server at startup: its "_comment"
key was read as a domain and refused as not a URL, while the test that
checks the example skipped it. Keys starting with an underscore are notes
now -- no mail domain starts with one -- and the example is also loaded
through the real parser in a test, so the two cannot disagree again.

Two new strings, in all nine catalogues.

stalwart-servers.example.json

@@ -17,9 +17,15 @@
     "JSON, a duplicate domain, or a value that is not an http(s) URL stops the",
     "server at startup rather than failing quietly at somebody's sign-in.",
     "",
+    "A value may instead be an object that also says where that server's own",
+    "administration is, for the link on ihasmail's Administration dashboard:",
+    "{\"url\": ..., \"adminUrl\": ...}. STALWART_ADMIN_URL is the same for the",
+    "default server. A listed domain without adminUrl gets no link, never the",
+    "default server's.",
+    "",
     "Docs: https://docs.ihasmail.org/configure/#several-stalwart-servers"
   ],
 
   "example.com": "https://mail.example.com",
-  "customer-b.test": "https://jmap.customer-b.test"
+  "customer-b.test": { "url": "https://jmap.customer-b.test", "adminUrl": "https://admin.customer-b.test" }
 }