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