[Cashtab] Separate scroll styling for portability

Summary: We want to use this on a new component to support agora partial trading. Move it so it can be edited in one place and support multiple components.

Test Plan:
npm test, npm start and modal scrollbars still look the same

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16924