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.jsx

@@ -1,48 +0,0 @@
-/*
- * 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>
-    );
-};