Default view of graphs: Display an average rate for all current builds per channel

[philipp-sumo]

similar to what was done on awsy, it would be great to have a graph depicting an average rate of all current builds per release channel (current was defined on awsy as all builds that were released within a time-window of 6 weeks prior to the reporting date).
this would provide a somewhat meaningful baseline, that is not affected by spikes at the start of a release cycle when the usage hours of the new build are still low