Converting styling of all internal apps, Settings and core ownCloud from em to px. 

The styling written in "em" that was causing various issues like https://github.com/owncloud/core/issues/801 and https://github.com/owncloud/core/issues/1747 to users who used different sizes in the browsers has been converted to the more convenient "px". I have tried to cover almost the the entire CSS of the core repository. I gave it a test and the layout seemed fine. I hope I haven't missed out something. So, someone can give this a hard test as a lot of styling is affected by this. 
cc // @jancborchardt @Raydiation  @eMerzh