✨ (admin) persist entities for stacked charts more often

[sophiamersmann]

• When creating a new chart, we want to sample a random set of entities
• But when editing a chart, we usually want to persist entities (unless it doesn't make sense)
• For stacked charts (StackedBar, StackedArea, StackedDiscreteBar), we were throwing away entities more often than necessary
• This PR implements two improvements:
  • Stacked charts persist a single entity and only sample new entities when the current selection is empty
  • StackedDiscreteBar charts are treated like other stacked charts – see above