Add PhotoList#item_before & PhotoList#item_after

Tested-by: Mark Somerville <mark@crocodile.is> [Arch, MRI 2.1.1]
Signed-off-by: Mark Somerville <mark@crocodile.is>
Add navigator to the photo page to browse the list

Tested-by: Mark Somerville <mark@crocodile.is> [Arch, MRI 2.1.1, Chromium 33, FF28]
Signed-off-by: Mark Somerville <mark@crocodile.is>