Progress towards better email notifications (#3911)

Working mail from notification.  Still requires testing/cleaning

Add tests around checkout notification.

This also removes the ability to check out an asset to a location|asset
that requires acceptance/a Eula.  For 4.1 we may think about how to
support such a thing, but at present it seems to make sense to only alow
such assets to be checked out to users, who can be responsible for the
items.

tests/unit/BaseTest.php

@@ -1,4 +1,5 @@
 <?php
+use App\Models\User;
 use Illuminate\Foundation\Testing\DatabaseMigrations;
 use Illuminate\Foundation\Testing\DatabaseTransactions;
 use Illuminate\Foundation\Testing\WithoutMiddleware;
@@ -11,4 +12,14 @@ class BaseTest extends \Codeception\TestCase\Test
         Artisan::call('migrate');
         factory(App\Models\Setting::class)->create();
     }
+
+    protected function signIn($user = null)
+    {
+        if (!$user) {
+            $user = factory(User::class)->states('superuser')->create();
+        }
+        Auth::login($user);
+
+        return $user;
+    }
 }