For price differences, use moment's duration

Instead of the duration library, use moment's duration to build the
duration string in the tracked items UI controller.  This was changed
because moment rounds the duration which makes it a bit easier to
interpret.
Calculate current price duration from today

Instead of calculating the duration that the current price has existed
from the previous price, calculate it from the difference of today till
it was established.
Display price history

Show the price history of an item within the item details page.  Also
minor UI updates.