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

src/applications/auth/controller/oauthregistration/PhabricatorOAuthDefaultRegistrationController.php

@@ -65,7 +65,7 @@ final class PhabricatorOAuthDefaultRegistrationController
         $errors[] = 'Username is required.';
       } else if (!PhabricatorUser::validateUsername($username)) {
         $e_username = 'Invalid';
-        $errors[] = 'Username must consist of only numbers and letters.';
+        $errors[] = PhabricatorUser::describeValidUsername();
       } else {
         $e_username = null;
       }