Use time range to calculate default granularity instead of options.intervalMs

[hulien22]

It seems as though options.intervalMs varies depending on the panel type. We currently use this value to calculate a granularity to set if no granularity is provided. However, this can be confusing to users since if they have the same query in two different panels (e.g. graph and singlestat), then they can get different values.
I propose using the time range to calculate what the interval should be ourselves, that way we do properly give the same results across all panels.