Convert to TypeScript

This needs the usual "I know the `app` element exists" annotation, plus
dealing with a non-existing /etc/hostname (in which case the watch will
return `null`, and we shouldn't poke that into a `string` state).

src/app.tsx

@@ -0,0 +1,48 @@
+/*
+ * This file is part of Cockpit.
+ *
+ * Copyright (C) 2017 Red Hat, Inc.
+ *
+ * Cockpit is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation; either version 2.1 of the License, or
+ * (at your option) any later version.
+ *
+ * Cockpit is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with Cockpit; If not, see <http://www.gnu.org/licenses/>.
+ */
+
+import React, { useEffect, useState } from 'react';
+import { Alert } from "@patternfly/react-core/dist/esm/components/Alert/index.js";
+import { Card, CardBody, CardTitle } from "@patternfly/react-core/dist/esm/components/Card/index.js";
+
+import cockpit from 'cockpit';
+
+const _ = cockpit.gettext;
+
+export const Application = () => {
+    const [hostname, setHostname] = useState(_("Unknown"));
+
+    useEffect(() => {
+        const hostname = cockpit.file('/etc/hostname');
+        hostname.watch(content => setHostname(content?.trim() ?? ""));
+        return hostname.close;
+    });
+
+    return (
+        <Card>
+            <CardTitle>Starter Kit</CardTitle>
+            <CardBody>
+                <Alert
+                    variant="info"
+                    title={ cockpit.format(_("Running on $0"), hostname) }
+                />
+            </CardBody>
+        </Card>
+    );
+};