[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/package.json

@@ -9,10 +9,10 @@
     "grunt-contrib-less": "~3.0.0",
     "grunt-contrib-uglify": "~5.0.1",
     "grunt-xmlmin": "~0.1.8",
-    "grunt-contrib-htmlmin": "~3.1.0",
     "grunt-contrib-watch": "~1.1.0",
     "grunt-eslint": "^23.0.0",
     "grunt-stylelint": "^0.16.0",
+    "grunt-image": "^6.4.0",
     "ionicons": "^5.5.4",
     "less": "^4.1.1",
     "less-plugin-clean-css": "^1.5.1",