[Cashtab] Copy OP_RETURN message button added to Tx collapse

Summary: Related to task T2167. Added button to Tx dropdown to copy OP_RETURN message if one is present.

Test Plan:
cd web/cashtab
npm start
if there are no Txs in TxHistory with an OP_RETURN msg, send one from another wallet
click a Tx with an OP_RETURN msg
click the Copy OP_RETURN button
observe the notification
ensure that the text copied to clipboard is correct

click a Tx with no OP_RETURN msg
observe that there is no Copy OP_RETURN button

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11313