Admins should be able to adjust histogram binning

[JG-QuarkNet]

Controls to independently adjust the bin widths of the 2-lepton and 4-lepton histograms should be added to the Admin panels.

These should be global, affecting all histograms of the given type. There's no need to set this parameter on a masterclass-by-masterclass basis or on a location-by-location basis.

Remember to include axis labels when changing bin widths.

Related to #17, "Review and revise Admin workflow"