[keyserver] Block usernames that could be valid ethereum addresses from being registered

Summary: Use isValidEthereumAddress(...) check to block registration with username that could be a valid ethereum address based on isValidEthereumAddress(...) check.

Test Plan:

1. Create user with random username ("hello") and make sure it works
2. Create user with possibly valid ethereum address ("0xe92d1a43df510f82c66382592a047d288f85226f") and make sure it does not work:

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6109