[Cashtab] Moved param alert above send button

Summary: Related to task T2328. Param alert message on send tab has been moved above the 'Send' button.

Test Plan:
cd web/cashtab && npm start
navigate to send tab
input an address that includes an amount param
observe that the param alert message appears above the send button

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11242