jsxc: Fix issues with jQuery >= 3.5.0

In testing and unstable, jQuery 3.5.1 is available. For security reasons, it
removes expanding of HTML code that JSXC relies on. Restore that functionality
as shown in jQuery Upgrade Guide.

Tests performed:

- Log in JSXC. Add a contact. In the contact list, the name of the contact is
shown properly.

- Clicking on the contact open a proper chat window.

- Using to two user accounts add each other to contact lists and start
conversation, the conversation show work properly.

- Authenticating each other and performing a secure conversation should work
properly.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/modules/jsxc/static/jsxc-plinth.js

@@ -43,6 +43,16 @@
 # THE SOFTWARE.
 */
 
+// Provide compatibility with jQuery Core >= 3.5 by expanding HTML code similar
+// to how it was done before jQuery Core < 3.5. This code was removed because it
+// is potentially insecure when the HTML code being parsed is coming from the
+// user input. See: https://jquery.com/upgrade-guide/3.5/ . JSXC >= 4.0 likely
+// does not need this.
+var rxhtmlTag = /<(?!area|br|col|embed|hr|img|input|link|meta|param)(([a-z][^\/\0>\x20\t\r\n\f]*)[^>]*)\/>/gi;
+jQuery.htmlPrefilter = function(html) {
+    return html.replace(rxhtmlTag, "<$1></$2>");
+};
+
 $(function() {
     const body = document.querySelector('body');
     const root = body.getAttribute('data-jsxc-root');
@@ -93,12 +103,13 @@ $(function() {
         formElements.prop('disabled', true);
         $('.submit').hide();
         $('.logout').show();
-    }
+    };
+
     var logged_out_state = function() {
         formElements.prop('disabled', false);
         $('.submit').show();
         $('.logout').hide();
-    }
+    };
 
     $(document).on('close.dialog.jsxc', function() {
         jsxc.debug('Event triggered close.dialog.jsxc');