[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>