Related to https://github.com/searxng/searxng/issues/1326
URL for the logo is referenced twice:
* in index.html for preloading: it contains the hash for cache busting (when static_use_hash: true)
* in searxng.min.css: to actually display the image. The URL doesn't contain the hash.
So the image preload actually loads twice the same image.
This commit removed the image preloading.
This is workaround: the real fix is to be able to use the URL with the hash in the CSS.
* update from ionicons-npm to ionicons ver.5
* drop the webfont built by grunt for icons
* built icons.html template for inlining icons with jinja2 into html
* update icon to use mostly the outline version
* add icons to categories and do not display them on mobile to save space
* remove all legacy ion icon font files from simple theme
* icons.html is added in this commit since make statc.build.restore requires git to know the file already
* cleanup error-dialog
