Modernize the code of the 2k11 backend theme

[onli]

The backend theme hasn't been kept up to date. While as far as I'm aware nothing is broken, in the spirit of enabling future development some aspects of it could be simplified. Things I saw:

• Remove the usage of Modernizer.js. It should be possible to replace the checks with custom code or just remove them, e.g. it's fine now to just assume that flexbox is always supported.
• Remove the checks (and custom CSS) for Internet Explorer 8
• Don't bundle the several javascript files in a plugins.js file, parallel downloads are okay now. The gruntify script can then also be removed.
• Remove accessifyhtml5.js, I think it's not needed anymore (and the repo archived)
• Check all other plugins whether they are actually used, and remove them if not. Some might also be replaceable with modern browser features (e.g. equal height -> css grid).
• Update the icons. But consider using SVG icons instead of just a new version of fontawesome.