luci-app-statistics: rearrange graph display

 - Add a per-plugin overview tab that contains the (non-detail) graphs of
   each plugin instance, similar to the old Lua based implementation
 - Numerically order plugin instances starting with numbers
 - Avoid multiple renderings of the same graphs
 - Fix legend of tcpconns graphs
 - Move cpufreq distribution and transition charts to detail tabs

Signed-off-by: Jo-Philipp Wich <jo@mein.io>