Raise an error if a user tries to register with an excessively long username

Summary: Fixes T2348. We should probably do some of this more broadly, but can tackle them one at a time as they arise, since many fields have no effective length limit.

Test Plan: {F54126}

Reviewers: btrahan, asherkin

Reviewed By: asherkin

CC: aran

Maniphest Tasks: T2348

Differential Revision: https://secure.phabricator.com/D6744