[OC 7] IE8 styling fixes

Fixes https://github.com/owncloud/core/issues/9678
Fixes https://github.com/owncloud/core/issues/9677

Please review and test in IE8: @MorrisJobke @DeepDiver1975 @icewind1991 

Note that I intentionally used the `.ie8` selector as I don't want to risk breaking anything else at this point of the release.
I could as well put `overflow-y:auto` in the main block, but I didn't want to risk breaking anything else. It did seem to work.