[Cashtab] Use new sendXec function to register aliases
Summary:
Stop using the legacy dependent registerNewAlias function and instead use the new sendXec function
After this is in, will have a separate diff to deprecate legacy transaction building functions that are no longer used.
Test Plan: npm test, change aliasEnabled flag to true, npm start, register an alias, confirm it is live on test server
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D15055