luci-app-statistics: add graphs for sqm-collect

luci-app-statistics: add graphs for sqm-collect

Add graphs for sqm data collected by 'sqm_collect.sh'

An overall sqm graph displays bandwidth (Kb/s), Backlog (Bytes) and
drops for each supported qdisc.

Sqm-cake supports cake's tin structure and displays many(!) graphs per
tin:

Bandwidth: Bandwidth (Kb/s), bandwidth threshold (Kb/s).
Latency: Target latency, Peak, Average, Sparse flow latency
Backlog v Drops: Backlog (Bytes), Drops, ECN marks, Ack-filter drops
Flow counts: Sparse flows, Bulk flows, Unresponsive flows

Fun for all the family :-)

Signed-off-by: Kevin Darbyshire-Bryant <redacted>