Make settings panels more modular and modern

Summary:
Currently, we have a hard-coded list of settings panels. Make them a bit more modular.

  - Allow new settings panels to be defined by third-party code (see {D2340}, for example -- @ptarjan).
  - This makes the OAuth stuff more flexible for {T887} / {T1536}.
  - Reduce the number of hard-coded URIs in various places.

Test Plan: Viewed / edited every option in every panel. Grepped for all references to these URIs.

Reviewers: btrahan, vrana, ptarjan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D3257

src/applications/people/controller/PhabricatorEmailVerificationController.php

@@ -51,7 +51,7 @@ final class PhabricatorEmailVerificationController
     $settings_link = phutil_render_tag(
       'a',
       array(
-        'href' => '/settings/page/email/',
+        'href' => '/settings/panel/email/',
       ),
       'Return to Email Settings');
     $settings_link = '<br /><p><strong>'.$settings_link.'</strong></p>';