[Cashtab][Pt 1/4] Move message signing and verification to new screen - Create icon & NavItem

Summary: Related to T2734. Message signing and verification collapses should be on their own screen. Part one of this diff includes creating the custom icon component and adding the new option to the hamburger nav menu

Test Plan:
cd web/cashtab && npm start
open the hamburger menu
observe that there is a new NavItem 'Sign & Verify Messages'

Per review feedback in D1241, styling has been fixed to center NavItem text.

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D12426