[cashtab] Menu style tweaks

Summary:
The slide in menu is appearing with some odd spacing and getting cut off on some mobile devices
Adding a few css tweaks to address this by adding scroll if the screen is too short

Test Plan: Preview the site and check the menu

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D17124