[WIP] Use Open Sans as typeface

Instead of letting the operating system decide we use font-face to deliver Open Sans for a consistent experience. This is work-in-progress as it needs to be tested on many different systems to make sure the text is shown everywhere. (Think different font formats etc.) There’s also fallbacks in place for other systems just if anything fails.

Please check @owncloud/designers 
cc for info @karlitschek