New layout and responsive CSS

This new design reflects a desire to using ISOworks as a full photo management
suite.

The srcset/sizes implementation is new in Chromium 38 and is a bit flaky
(sometimes, images are automatically reloaded on resize, more often they're
not).

Tested-by: Mark Somerville <mark@crocodile.is> [Chromium 38]
Signed-off-by: Mark Somerville <mark@crocodile.is>