[Cashtab] Cosmetic update for message signing component

Summary:

• Reduced whitespace between the Send XEC and Sign Message components based on user feedback.
• Added Signature label

Test Plan:

• npm start
• verify sign message UI positioning across chrome and firefox
• npm run extension
• very UI positioning in extension mode

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D10434