remove shadow effect from icon, fix add icon

All shadow effects have been removed from core icons: https://github.com/owncloud/core/pull/9381

So the apps should follow. :) Please review @owncloud/designers @Raydiation @cosenal 

Also, the add icon was not pixel-perfect. Fixed that by taking the add icon from core. @Raydiation the News app duplicates a lot of icons from core – can you instead use the classes from core for that? Like ```.icon-download```, ```.icon-add``` etc?