Allow usernames to include ".", "-" and "_"

Summary:
See T1303, which presents a reasonable case for inclusion of these characters in valid usernames.

Also, unify username validity handling.

Test Plan: Created a new user with a valid name. Tried to create a new user with an invalid name. Ran unit tests.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1303

Differential Revision: https://secure.phabricator.com/D2651