Better heatmap intensity system

[manuboojuk]

Describe the solution you'd like
The intensities on the heatmap should take into account the weights of the corresponding assessments and for high weight assessments, the intensity should leak into days prior to that deadline in order to better represent workload intensity.

Describe Possible Solutions
This issue should depend on the 'Change heatmap package' issue since with the current package that we're using you probably can't implement a feature like this so we would need to switch the package we're using for the heatmaps first.

Additional context
This issue is for the winter 2022 upgrade of Viaplanner.