Add external authenticator (social login) API

Allows registrations to be completed with a pre-confirmed email address
and no password.

migrations/2015_02_24_000000_create_email_tokens_table.php

@@ -23,8 +23,8 @@ class CreateEmailTokensTable extends Migration
     {
         $this->schema->create('email_tokens', function (Blueprint $table) {
             $table->string('id', 100)->primary();
-            $table->integer('user_id')->unsigned();
             $table->string('email', 150);
+            $table->integer('user_id')->unsigned();
             $table->timestamp('created_at');
         });
     }

migrations/2015_09_15_000000_make_email_tokens_user_id_column_nullable.php

@@ -0,0 +1,40 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+use Flarum\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+
+class MakeEmailTokensUserIdColumnNullable extends Migration
+{
+    /**
+     * Run the migrations.
+     *
+     * @return void
+     */
+    public function up()
+    {
+        $this->schema->table('email_tokens', function (Blueprint $table) {
+            $table->integer('user_id')->unsigned()->nullable()->change();
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     *
+     * @return void
+     */
+    public function down()
+    {
+        $this->schema->table('email_tokens', function (Blueprint $table) {
+            $table->integer('user_id')->unsigned()->change();
+        });
+    }
+}