Disabling send for SLPA tokens

Summary: Disabling Send for SLPA tokens pending upgrades to the back end

Test Plan:

npm start

Navigate to localhost:3000. Try to send an SLPA token from your wallet. The Send button will be greyed out, and the Send page will include the message "Sending SLPA tokens is currently disabled"

Reviewers: majcosta, alcipir, #bitcoin_abc

Reviewed By: alcipir

Differential Revision: https://reviews.bitcoinabc.org/D8583