Improve errors associated with adding new login providers

Summary:
Ref T1536.

  - When users try to add a one-of provider which already exists, give them a better error (a dialog explaining what's up with reasonable choices).
  - Disable such providers and label why they're disabled on the "new provider" screen.

Test Plan:
{F47012}

{F47013}

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6256

src/view/AphrontDialogView.php

@@ -14,6 +14,13 @@ final class AphrontDialogView extends AphrontView {
   private $headerColor = PhabricatorActionHeaderView::HEADER_DARK_GREY;
   private $footers = array();
   private $isStandalone;
+  private $method = 'POST';
+
+
+  public function setMethod($method) {
+    $this->method = $method;
+    return $this;
+  }
 
   public function setIsStandalone($is_standalone) {
     $this->isStandalone = $is_standalone;
@@ -159,7 +166,7 @@ final class AphrontDialogView extends AphrontView {
 
     $form_attributes = array(
       'action'  => $this->submitURI,
-      'method'  => 'post',
+      'method'  => $this->method,
       'id'      => $this->formID,
     );