[Cashtab] Copy own address from Sign Message collapse in Send.js

Summary: Related to T2362. Part of the user's request was to add the ability to copy one's own address from the Sign Message collapse in Send.js.

Test Plan:
cd web/cashtab && npm start
navigate to send screen
expand the Sign Message collapse
click on the Addres input
observe that current wallet's address is copied to clipboard
observe the copy success notification

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11559