[fix] Optimize SVG for WEB usage / CSP 'style-src self'

- Replace grunt-contrib-htmlmin by grunt-image [1].

- Activate svgo's [2] convertStyleToAttrs to make the HTML inline SVGs
  compoliant to the CSP policy [3]::

    Content-Security-Policy: style-src self;

[1] https://www.npmjs.com/package/grunt-image
[2] https://github.com/svg/svgo
[3] https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/style-src

Closes: https://github.com/searxng/searxng/issues/502

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

searx/static/themes/simple/svg4web.svgo.js

@@ -0,0 +1,19 @@
+/**
+ * @license
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ *
+ * svgo config: Optimize SVG for WEB usage
+ */
+
+module.exports = {
+  plugins: [
+    {
+      name: 'preset-default',
+    },
+    // make diff friendly
+    'sortAttrs',
+    // Optimize SVG for WEB usage
+    'convertStyleToAttrs',
+    'removeXMLNS'
+ ],
+};