[lib] Limit username input length

Test Plan: Try to create accounts with username length: 5, 6, 191, 192 characters - 5 and 192 should fail, other succeed.

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, atul, subnub

Differential Revision: https://phabricator.ashoat.com/D876