Show "Go to top" on quick scroll up in subreddit view

[TheNightmanCodeth]

Since the appbar is gone, a user should be able to get back to the top of the list quickly without the ability to tap on the appbar. I'm thinking a pill-shaped message can be displayed from the top of the list when it is scrolled quicky upwards.

To implement this we'll need to make a CustomScrollView. If anyone reading this would like to give it a go, the docs are here.