Provide contextual help on auth provider configuration

Summary:
Ref T1536.

  - Move all the provider-specific help into contextual help in Auth.
  - This provides help much more contextually, and we can just tell the user the right values to use to configure things.
  - Rewrite account/registration help to reflect the newer state of the word.
  - Also clean up a few other loose ends.

Test Plan: {F46937}

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6247

src/applications/auth/controller/config/PhabricatorAuthEditController.php

@@ -224,6 +224,12 @@ final class PhabricatorAuthEditController
           ->addCancelButton($cancel_uri)
           ->setValue($button));
 
+    $help = $provider->getConfigurationHelp();
+    if ($help) {
+      $form->appendChild(id(new PHUIFormDividerControl()));
+      $form->appendRemarkupInstructions($help);
+    }
+
     $crumbs = $this->buildApplicationCrumbs();
     $crumbs->addCrumb(
       id(new PhabricatorCrumbView())