Completed high-resolution sprites

Fixed resolveUrl for null URL’s
Fixed appearance of folder icons underneath sub-folder favicons (in
hi-res mode)
Fixed height of article footers on mobile devices
Added display of white underlay while sidebar is open
Moved sign in button closer to center of page

Removed logo from intro page
Minor styling changes to shortcut viewer
Fixed subscription name overflow bug for floating sidebars
Removed underlay from sidebar (again)