Add scroll-hint affordance to long `FilterList`s, `GroupMenu`

[lyzadanger]

As noted in https://github.com/hypothesis/client/pull/5268, there are a few limitations to the current styling of long FilterList overflows:

• There is no visual affordance, other than "we hope that one of the entries is truncated in a way to suggest it" that the overflow is scrollable
• This overflow approach should also be applied to the GroupMenu, which currently can be unusably long in shorter viewports

[Nitesh639]

@lyzadanger, I think this issue is resolved. We should close this issue.

[robertknight]

It is still true that we don't have an explicit hint for scrollability as we do in eg. tables where we have a shadow. I do think the control nevertheless "looks" scrollable, to the extent that we can probably get away without this.