Add Predictive Intervals to Validation Plots

[trobacker]

We'd like to add predictive intervals to the validations plots (at least once the validation period of ~90days has passed). To be completed after #746 is finished.

Tasks:

• Generate predictive intervals for the observed clade frequencies based on observed counts with multinomial sampling after the evaluation period overlayed with validation data (currently, the orange points).

[trobacker]

Extension of #788