[Cashtab] Patched vertical alignment of NavItems in hamburger menu

Summary: Related to T2775. Patched vertical alignment of NavItem text and icons.

Test Plan:
cd web/casthab && npm start
open the hamburger menu
observe that the NavItem text and NavItem icons are centered vertically.

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D12439