Store imagare user preferences in a browser cookie

This avoids making a call to the server to fetch the preferences
everytime a change or diff screen is viewed.

The cookie expires once a day so that new defaults will be picked up.

Change-Id: I73871efa32f3c8b6b7b84aa6b41864c766eb537c
Signed-off-by: Edwin Kempin <ekempin@google.com>