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

scripts/user/account_admin.php

@@ -29,8 +29,8 @@ if (!strlen($username)) {
 }
 
 if (!PhabricatorUser::validateUsername($username)) {
-  echo "The username '{$username}' is invalid. Usernames must consist of only ".
-       "numbers and letters.\n";
+  $valid = PhabricatorUser::describeValidUsername();
+  echo "The username '{$username}' is invalid. {$valid}\n";
   exit(1);
 }