[Cashtab] Replace hardcoded param with variable

Summary: Removed two insances of 5.5 hardcoded in Send.js and replaced them with fromSmallestDenomination(clurrency.dustSats). Related to task T2107.

Test Plan:
npm web/cashtab && npm start
Use the reply message link and ensure that the amount input on the send form is 5.5.
Input an amount less than 5.5 in the amount input and ensure the error message indicates that you need to send at least 5.5 XEC.

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D10729