[Cashtab] Clear amount & address fields on send

Summary: Added a user submitted function which clears the address and amount fields when the send button is clicked. Appears to work when sending XEC, sending a message, or replying to a message. Related to task T2075.

Test Plan:
cd web/cashtab && npm start
navigate to send tab
send XEC
from another wallet:
send a message
from local wallet:
reply to message

ensure transactions go through and that the address and amount fields are cleared on click

Reviewers: bytesofman, #bitcoin_abc, emack

Reviewed By: bytesofman, #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D10665