uci-app-vnstat2: refactoring

This is a huge change to the already existing vnstat2 LuCI module with some
improvements and some new features.

"Graphs Page":

  - Implementation of refreshing the graphs.

  - Only in the UCI configuration existing interfaces will be shown in any
    case. Before this change, all interfaces existing in the database were
    shown.

  - Introduced a button to clear the statistics for all interfaces (in fact
    this is removing and adding again the interfaces from/to the database).

    Before clearing the data a confirmation dialog is shown.

  - Show user hint if service is not running, so no updating of graphs.

  - "Error image" for a graph which can not be loaded

General:

  - Updated translations, added missing translations for ./po/de

  - Renamed the menu entry from "vnStat Traffic Monitor" to "Traffic Monitor" only

Signed-off-by: Helge Mader <ma@dev.tdt.de>