Optimize header grid for tablet

Use boostrap's col-md-* in header to separate tablet portrait and landscape. LP#15202477
Add asset toolbar plus better icon handling

Download and share is now in a justified button toolbar. Both use Boostrap dropdowns and svg icons.

Create a  mixin for various (svg) icons.

LP#15202477
Update style for asset view

Includes some icons, bold fonts, inline lists (categories class) and full width Google map

LP#15202477
Merge pull request #23 from NationalMuseumofDenmark/FrontendDesign#15202477

Frontend design#15202477