Optimize scrollable lists with `SliverFixedExtentList` / `SliverVariedExtentList`

[navaronbracke]

Since there are slivers for more efficient list scrolling, we should make use of the available widgets.

See

• https://api.flutter.dev/flutter/widgets/SliverFixedExtentList-class.html
• https://api.flutter.dev/flutter/widgets/SliverVariedExtentList-class.html