[identity] Update invalid username check on Identity service
Summary:
- check that the username is valid (no invalid special characters, min length 1, max length 191, starts with alphanumeric character)
- return invalid username error instead of username reserved error if username is a valid eth address
Test Plan:
- unit tests
- used commtest to try registering various invalid usernames, confirmed that i got an invalid username response from identity each time
Reviewers: bartek, michal, will
Reviewed By: bartek
Subscribers: ashoat, tomek
Differential Revision: https://phab.comm.dev/D11329