[Cashtab][Pt 3/4] Move message signing and verification to new screen - create new SignVerifyMsg component

Summary: Related to T2734, depends on D12428. Message signing and verification collapses should be on their own screen. Part 3 of this diff is the addition of the new SignVerifyMsg directory / component. Routes have been added in extension/src/components/App.js and web/cashtab/src/components/App.js. Per review feedback in D12417, cashtabSettings and changeCashtabSettings are not passed through App.js, but pulled from context the same way as is done on other screens.

Test Plan:
cd web/cashtab && npm start
navigate to Send.js
observe that the Sign and verify message collapses are no longer there
expand the hamburger menu
click Sign and Verify
observe that the new component is rendered at the route /signverifymsg

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D12430