[fix] ionic.io/ionicons - add back missing class="ionicon"

Seems to me svg2jinja added the class in the past .. but no longer in new builds
/ this patch adds the class back by using addAttributesToSVGElement [1].

To test this patch use:

    $ ./manage themes.simple
    $ cat searx/templates/simple/icons.html

and check `class="ionicon"` is in the outer `<svg ..>` tags.

[1] https://svgo.dev/docs/plugins/add-attributes-to-svg-elements/
[2] https://ionic.io/ionicons

Closes: https://github.com/searxng/searxng/issues/3383
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searx/static/themes/simple/gruntfile.js

@@ -218,7 +218,7 @@ module.exports = function (grunt) {
               name: "addAttributesToSVGElement",
               params: {
                 attributes: [
-                  { "aria-hidden": "true" }
+                  { "class": "ionicon", "aria-hidden": "true" }
                 ]
               }
             }