Proper collapsing behavior for index tiles

Index tiles are now displayed in full on large devices with no option
of collapsing. On mobile devices (or when the browser window is resized
to mobile-like size) tiles are automatically collapsed and an
expand/collapse link appears in header.

collapse-xs: Manages collapsing/expanding the tiles as appropriate.
Changes chevron icon direction in link.
base: Added link to collapse-xs.
index: Converting all current tiles to collapsable format.