Server: icon-white class doesn't work on IE11

[juliushaertl]

Since IE11 is still supported we cannot use the .icon-white class as added in #7319, since filter:invert(100%); is not supported: https://caniuse.com/#search=invert

A possible solution would be to use something like https://github.com/shekhardesigner/InvertImages to provide a fallback solution for IE11. We could even simplify the script a bit: https://github.com/shekhardesigner/InvertImages/blob/master/jquery.invertImages.js

@nextcloud/designers @pixelipo

[jancborchardt]

cc @skjnldsv @nextcloud/designers since we talked about this in the #9862 accessibility pull request.

[MorrisJobke]

Fix is in #9984

[juliushaertl]

Not yet fixed, #9984 is the foundation of it, we still need to get rid of the invert filter and load the icons with white color for the icon-white class. I'll have a look.