mobile: smaller images for smaller screens, fit 2 to width
Pictures: make header scroll away on mobile
Pictures: fix 'Picture view' button position
Pictures: change notice to 'shared by username' as in Files
Pictures: change 'Picture view' and 'File list' toggles to icons
fix missing file extension
Merge pull request #1607 from owncloud/mobile-style

Mobile style for public links: files, folders, picture sets