luci-app-statistics: reorganise menu items

Reorganise the menu items of the Luci statistics module:

* Re-label "Collectd" to "Setup" and place it after the "Graphs" items.
  - "Graphs" is used much more frequently so it should be first.
  - Change "Collectd" menu item to "Setup" to clarify things.

* Re-label "System plugins" to "General plugins" to change sorting.
  "Network / Output / System" changes to "General / Network / Output"
  that is more logical order for the items.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>