Unify logic for username validation

Summary: Revisit of D1254. Don't require lowercase, just standardize the logic.
The current implementation has nonuniform logic -- PeopleEditController forbids
uppercase.

Test Plan: Ran unit tests, see also D1254.

Reviewers: btrahan, jungejason, aran

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D1415