[Cashtab] Clear form data on eToken send

Summary: Added function which clears the address and amount fields when sending eTokens. Related to task T2106.

Test Plan:
cd web/cashtab && npm start
navigate to send-token
send token and ensure form data is cleared

Reviewers: emack, bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D10703