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/views/admin/__tests__/admin-dashboard.test.tsx

@@ -111,3 +111,40 @@ describe("the Administration dashboard", () => {
     expect(cards()[4]).toEqual(["Received", "—", "Could not be loaded"]);
   });
 });
+
+describe("the pointer to Stalwart's own administration", () => {
+  let host: HTMLDivElement;
+  let root: Root;
+  beforeEach(() => {
+    host = document.createElement("div");
+    document.body.appendChild(host);
+    root = createRoot(host);
+  });
+  afterEach(async () => {
+    await act(async () => root.unmount());
+    host.remove();
+  });
+  const renderWith = async (adminUrl: string | null) => {
+    useSession.setState({ session: { capabilities: {}, accounts: {}, primaryAccounts: {}, username: "[email protected]", ihasmail: { permissions: HELPDESK, server: { edition: "enterprise", adminUrl } } } as unknown as JmapSession });
+    const { hook } = memoryLocation({ path: "/admin" });
+    await act(async () => {
+      root.render(<Router hook={hook}><AdminDashboard /></Router>);
+    });
+    await act(async () => {});
+  };
+
+  it("names it, and links it where the operator has said where it is", async () => {
+    await renderWith("https://admin.example.com");
+    const note = host.querySelector(".admin-dashboard-note")!;
+    expect(note.textContent).toContain("Stalwart's own administration");
+    const link = note.querySelector("a")!;
+    expect(link.getAttribute("href")).toBe("https://admin.example.com");
+    expect(link.getAttribute("rel")).toBe("noopener noreferrer");
+  });
+
+  it("names it without a link where nobody has", async () => {
+    await renderWith(null);
+    expect(host.querySelector(".admin-dashboard-note")?.textContent).toContain("Stalwart's own administration");
+    expect(host.querySelector(".admin-dashboard-note a")).toBeNull();
+  });
+});