Added initial class for displaying invisible chars

Summary:
Fixes T11586. First pass at a class for displaying invisible characters. Still need to:
- Write a couple unit tests
- Add some styling to the .invisible-special spans
- Actually start using the class when displaying form errors to users

Currently this makes the string `"\nab\x00c\x01d\te\nf"` look like:

{F1812711}

Test Plan:
Unit tests all pass and run in <1ms:

{F1812998}

Reviewers: epriestley, #blessed_reviewers, chad

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley, yelirekim

Maniphest Tasks: T11586

Differential Revision: https://secure.phabricator.com/D16541

src/applications/auth/controller/PhabricatorAuthRegisterController.php

@@ -89,13 +89,14 @@ final class PhabricatorAuthRegisterController
       // user expectation and it's not clear the cases it enables are valuable.
       // See discussion in T3472.
       if (!PhabricatorUserEmail::isAllowedAddress($default_email)) {
+        $debug_email = new PHUIInvisibleCharacterView($default_email);
         return $this->renderError(
           array(
             pht(
               'The account you are attempting to register with has an invalid '.
               'email address (%s). This Phabricator install only allows '.
               'registration with specific email addresses:',
-              $default_email),
+              $debug_email),
             phutil_tag('br'),
             phutil_tag('br'),
             PhabricatorUserEmail::describeAllowedAddresses(),