[alias-server] Do not use toLowerCase()

Summary:
T3060

Alias strings are now validated by the isValidAliasString function. toLowerCase is not a practical way to handle potential mixed input. Such input is now disabled.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13711