[WIP] JS Extender API foundation (#1468)

* Run extenders exported by extensions
* Add some basic extenders
* Patch Mithril as the very first thing so extension code can run safely
* Load the payload into the app before booting extensions
* Setup default routes before booting extensions

views/frontend/app.blade.php

@@ -30,7 +30,7 @@
     @if ($allowJs)
       <script>
         document.getElementById('flarum-loading').style.display = 'block';
-        var flarum={extensions:{}};
+        var flarum = {extensions: {}};
       </script>
 
       @foreach ($jsUrls as $url)
@@ -42,7 +42,9 @@
         @if (! $debug)
         try {
         @endif
-          flarum.core.app.boot(@json($payload));
+          flarum.core.app.load(@json($payload));
+          flarum.core.app.bootExtensions(flarum.extensions);
+          flarum.core.app.boot();
         @if (! $debug)
         } catch (e) {
           window.location += (window.location.search ? '&' : '?') + 'nojs=1';