Bugfix: Remove fixed login logo width

The logo displayed on login/error pages had a fixed width and height, wich was fine for 
the provided ownCloud logo but produced suboptimal results when using files with other 
width/height ratios. By changing this to max-width & max-height we allow for other files 
while keeping their proper aspect ratio.

https://github.com/owncloud/owncloud-design-system/pull/1207
