Show gradient on scrollable Select elements

[adamb70]

Changes

Fixes #2050. This makes scrollable select inputs more obvious on browsers with hidden scrollbars.

Adds an extended Ant Select element that should be a drop-in replacement.

I've only applied it to the Filters, Breakdowns, and Path Start Points for now, wasn't sure if anywhere else needed it.

First time using React, so let me know if I made any atrocities!

Checklist

• All querysets/queries filter by Organization, Team, and User (if this PR affects ANY querysets/queries).
• Django backend tests (if this PR affects the backend).
• Cypress end-to-end tests (if this PR affects the frontend).

[Twixes]

Hey, thanks a lot, this is nice! I added a gradient at the top too for symmetry (when not at the top of the list).

Just one idea for improvement is animating this – a ~200ms transition could make this look much smoother.