Run local extenders before booting service providers

We still need to discuss the priority of local extenders vs. those
from enabled extensions, but let's first fix the actual bug.

Refs #1708.

src/Foundation/InstalledSite.php

@@ -146,12 +146,12 @@ class InstalledSite implements SiteInterface
 
         $laravel->register(ExtensionServiceProvider::class);
 
-        $laravel->boot();
-
         foreach ($this->extenders as $extension) {
             $extension->extend($laravel);
         }
 
+        $laravel->boot();
+
         return $laravel;
     }