[Cashtab[Alias] Clear registration input after broadcast

Summary: Clear the input field for the alias registration component upon broadcast.

Test Plan:
activate alias flag in config
npm start
navigate to Alias.js, register a new alias and observe the input field being cleared afterwards

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D14610