[Cashtab] Fix formatting issue for Cashtab msgs containing long strings

Summary: Related to T2755. Long unbroken strings were pushing out the OpReturnType div beyond the boundaries of the app.

Test Plan:
cd web/cashtab && npm start
ensure there is a tx with a long unbroken string in recent tx history
observe that the message wraps instead of breaking out of the containing div

Reviewers: bytesofman, emack, #bitcoin_abc

Reviewed By: bytesofman, emack, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D12364