[Cashtab] Parse amount param in XEC

Summary: Changed the handling of the amount param in Cashtab to ensure consistency with other XEC wallets. Related to task T2338.

Test Plan:
cd web/cashtab && npm start
navigate to send screen
input an eCash address with an amount param including decimals
ensure tx goes through
input an eCash address with an amount param without decimals
ensure tx goes through

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11403