Prevent tests from writing to window hash
Simplify `modelToDate` for sparkline graphs

We always want to plot against `_timestamp` so we can remove the code overhead of checking against the periods.
Prevent sending backdrop start_at and end_at dates with a duration

Backdrop throws an error if sent all three of start_at, end_at and duration parameters, so remove duration if the start and end dates are set.
Enable date pickers for remaining graph modules