luci-app-nlbwmon: Fixed incorrect period parameter/display

Switch from using the unreliable string data constructor syntax to
a more explicit year / month / day variant and pass through the
period value as-is in order to ensure that the proper start date
is requested from the backend.

This should fix issues with prevent the display of older data
periods in the nlbwmon gui.

Ref: https://github.com/jow-/nlbwmon/issues/25
Ref: https://github.com/openwrt/luci/pull/2496
Suggested-by: Aktanusa <chanam.geo@yahoo.com>
[squashed commits, add a more thorough commit message]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>