Adjust variant rollout by drag-and-drop

[Tymek]

Describe the feature request

Users should be able to adjust variant rollout by dragging percentage slider, not only with numeric inputs.

Background

If you go to Feature -> (open environment) -> Add strategy -> Variants, and add multiple variants, you have a chart on the bottom, that's not two-way interactive.

Solution suggestions

Add (invisible?) handles, that change cursor type when hovered, and allow you to change percentage.