[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