[fix] improve OpenSearch description

Some HTTP-Clients do have issues with the ``opensearch.xml`` from SearXNG
(related [1][2]) while other OpenSearch descriptions[3] (e.g. from qwant) work
flawles.

Inspired by the OpenSearch description from qwant and with informations from the
specification[4] the ``opensearch.xml`` has been *improved*.

- convert `<Url>` methods from lower case to upper case (`POST`|`GET`)
- add `<moz:SearchForm>` and `xmlns:moz="http://www.mozilla.org/2006/browser/search/"`
- add `<Query role="example" searchTerms="SearXNG" />`  [4]

  OpenSearch description documents should include at least one Query element of
  `role="example"` that is expected to return search results. Search clients may
  use this example query to validate that the search engine is working properly.

- modified `<LongName>` to SearXNG
- modified `<Description>` the word 'hackable' scares uninitiated users and was removed
- add the `type="image/png"` to `<Image>`

Test can be done by::

    make run

Visit http://127.0.0.1:8888/ and add the search engine to your WEB-Browser /
test with different WEB-Browser from desktop and Smartphones (are there any iOS
user here, please test on Safari and Chrome).

[1] https://app.element.io/#/room/#searxng:matrix.org/$xN_abdKhNqUlgXRBrb_9F3pqOxnSzGQ1TG0s0G9hQVw
[2] https://github.com/searxng/searxng/issues/431
[3] https://developer.mozilla.org/en-US/docs/Web/OpenSearch
[4] https://github.com/dewitt/opensearch/blob/master/opensearch-1-1-draft-6.md#the-query-element

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

tests/unit/test_webapp.py

@@ -244,7 +244,9 @@ class ViewsTestCase(SearxTestCase):
     def test_opensearch_xml(self):
         result = self.app.get('/opensearch.xml')
         self.assertEqual(result.status_code, 200)
-        self.assertIn(b'<Description>a privacy-respecting, hackable metasearch engine</Description>', result.data)
+        self.assertIn(
+            b'<Description>SearXNG is a metasearch engine that respects your privacy.</Description>', result.data
+        )
 
     def test_favicon(self):
         result = self.app.get('/favicon.ico')