Use duration to calculate status duration

Use the duration library to calculate the duration of time it takes to
update the prices within the status.  Instead of storing the status
update duration in seconds, store the status update start and end time.
 Then in the status-ui controller, use duration to calculate the
duration of time it took.
Enable mock URIs

To improve ease of testing, allow for a mock uri (“mock_uri”) which
will (if the TEST environment is enabled) generate a mock price,
category, and name of a tracked item.
Include price durations in tracked-items-ui

Add the duration the price held as the best price for the current and
past prices.  If it is the first price, this value is null.  Because of
yet another attribute to “currentPrice”, make it an object.