[Cashtab] Fix OpReturnMsg overflow

Summary: Related to T2621. Fixed styling issue in the OpReturnType styled-component so that no height property is applied unless hideMessagesFromUnknownSenders is true.

Test Plan:
cd web/cashtab && npm start
ensure there are outgoing & incoming Txs with messages in recent Tx history
add one of the wallets of a received message to contact list to trigger Reply button.

in devtools: set window dimensions to mobile

observe that the message content & reply button do not overflow at any size.

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11891