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.

web/src/locales/de.ts

@@ -147,6 +147,8 @@ export const catalog: Catalog = {
     "The numbers your role can see, as the server reports them.": "Die Zahlen, die Ihre Rolle sehen darf, so wie der Server sie meldet.",
     "Nothing to show": "Nichts anzuzeigen",
     "Could not be loaded": "Konnte nicht geladen werden",
+    "Detailed metrics, the delivery queue, logs and server settings are in Stalwart's own administration.": "Detaillierte Metriken, die Zustellwarteschlange, Protokolle und Servereinstellungen finden Sie in der Verwaltung von Stalwart selbst.",
+    "Open Stalwart admin": "Stalwart-Verwaltung öffnen",
     "User": "Benutzer",
     "Administrator": "Administrator",
     "Custom role": "Eigene Rolle",