[Cashtab] Improve Sign and Verify collapse labels

Summary: Related to T2793. Collapse panels on signverifymsg screen should simply read "Sign" and "Verify".

Test Plan:
cd web/cashtab && npm start
navigate to /signverifymsg
observe that collapse labels simply read "Sign" and "Verify"

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D12534