network-flow(donut): say how long the period being compared against was

"Previous" leaves the reader to work out what it is previous to. The window
is whatever the dashboard's time filter resolved to, so the column now names
it - "Prev 4 h" against a four-hour frame - and follows the frame when it
changes rather than stating a duration someone hard-coded once.

The chart has no notion of time and the legend holds no arithmetic, so
neither works the label out: the widget does, from the window the response
reports, and hands it down as a string. The breakdown of the remainder heads
its own comparison the same way, since the two tables describe the same two
windows.

CMK-37877

Change-Id: I13fe1888e3bbe9f43181d5aca40d632f205b0757