Support invites in the registration and login flow

Summary:
Ref T7152. This substantially completes the upstream login flow. Basically, we just cookie you and push you through normal registration, with slight changes:

  - All providers allow registration if you have an invite.
  - Most providers get minor text changes to say "Register" instead of "Login" or "Login or Register".
  - The Username/Password provider changes to just a "choose a username" form.
  - We show the user that they're accepting an invite, and who invited them.

Then on actual registration:

  - Accepting an invite auto-verifies the address.
  - Accepting an invite auto-approves the account.
  - Your email is set to the invite email and locked.
  - Invites get to reassign nonprimary, unverified addresses from other accounts.

But 98% of the code is the same.

Test Plan:
  - Accepted an invite.
  - Verified a new address on an existing account via invite.
  - Followed a bad invite link.
  - Tried to accept a verified invite.
  - Reassigned an email by accepting an unverified, nonprimary invite on a new account.
  - Verified that reassigns appear in the activity log.

{F291493}
{F291494}
{F291495}
{F291496}
{F291497}
{F291498}
{F291499}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7152

Differential Revision: https://secure.phabricator.com/D11737

src/applications/auth/controller/PhabricatorAuthStartController.php

@@ -109,14 +109,21 @@ final class PhabricatorAuthStartController
       }
     }
 
+    $invite = $this->loadInvite();
+
     $not_buttons = array();
     $are_buttons = array();
     $providers = msort($providers, 'getLoginOrder');
     foreach ($providers as $provider) {
-      if ($provider->isLoginFormAButton()) {
-        $are_buttons[] = $provider->buildLoginForm($this);
+      if ($invite) {
+        $form = $provider->buildInviteForm($this);
       } else {
-        $not_buttons[] = $provider->buildLoginForm($this);
+        $form = $provider->buildLoginForm($this);
+      }
+      if ($provider->isLoginFormAButton()) {
+        $are_buttons[] = $form;
+      } else {
+        $not_buttons[] = $form;
       }
     }
 
@@ -159,6 +166,11 @@ final class PhabricatorAuthStartController
     $login_message = PhabricatorEnv::getEnvConfig('auth.login-message');
     $login_message = phutil_safe_html($login_message);
 
+    $invite_message = null;
+    if ($invite) {
+      $invite_message = $this->renderInviteHeader($invite);
+    }
+
     $crumbs = $this->buildApplicationCrumbs();
     $crumbs->addTextCrumb(pht('Login'));
     $crumbs->setBorder(true);
@@ -167,6 +179,7 @@ final class PhabricatorAuthStartController
       array(
         $crumbs,
         $login_message,
+        $invite_message,
         $out,
       ),
       array(