Fix issue around "Class 'Grav\Plugin\Shortcodes\Shortcode' not found".

This patch fixes #4.

Context:

    Before this patch (after 4.2.0 of shortcode-core), I could
    not run Grav (bin/grav and webserver) anymore.

    Indeed, I got "Class 'Grav\Plugin\Shortcodes\Shortcode' not found"
    as error and asked for help on the discord server of the
    Grav Core Team.

    @mahagr & @rhukster tried to help me with a patch+release
    (shortcode-core 4.2.1) but it still did not solve the issue.
    So I decided to fix it myself.

Resolution:

    I first followed the new structure by moving the shortcodes classes
    inside the `classes` directory.
    Same for `classes/SSEShortcode.php` which I moved to
    `classes/plugin/SSEShortcode.php`.

    In the `static-social-embeds.php` file, I just fixed the autoloader.
    In particularly the `getSubscribedEvents()` method. From now,
    `classes/plugin/SSEShortcode.php` is only loaded when we are not
    in the admin section.

    After, applying and testing those changes, onto my fresh development
    environment (latest grav-admin+docker-compose), everything worked
    back properly.

static-social-embeds.php

@@ -1,4 +1,5 @@
 <?php
+
 namespace Grav\Plugin;
 
 use Grav\Common\Assets;
@@ -23,24 +24,36 @@ class StaticSocialEmbedsPlugin extends Plugin
      */
     public static function getSubscribedEvents()
     {
-        require_once __DIR__ . '/vendor/autoload.php';
-        require_once __DIR__ . '/classes/SSEShortcode.php';
-
         return [
-            'onPluginsInitialized' => ['onPluginsInitialized', 0]
+            'onPluginsInitialized' => [
+                ['autoload', 100001],
+                ['onPluginsInitialized', 0]
+            ],
         ];
     }
 
+    /**
+     * [onPluginsInitialized:100000] Composer autoload.
+     *
+     * @return ClassLoader
+     */
+    public function autoload()
+    {
+        return require __DIR__ . '/vendor/autoload.php';
+    }
+
     /**
      * Initialize the plugin
      */
     public function onPluginsInitialized()
     {
-        // Don't proceed if we are in the admin plugin
         if ($this->isAdmin()) {
+            // Don't proceed if we are in the admin plugin
             return;
         }
 
+        require_once __DIR__ . '/classes/plugin/SSEShortcode.php';
+
         // Enable the main event we are interested in
         $this->enable([
             'onShortcodeHandlers' => ['onShortcodeHandlers', 0],
@@ -65,18 +78,15 @@ public function onAssetsInitialized()
         /** @var $assets Assets */
         $assets = $this->grav['assets'];
 
-        if ($this->config->get('plugins.static-social-embeds.include_font_awesome_5', true))
-        {
+        if ($this->config->get('plugins.static-social-embeds.include_font_awesome_5', true)) {
             $assets->add('https://use.fontawesome.com/releases/v5.1.0/css/all.css');
         }
 
-        if ($this->config->get('plugins.static-social-embeds.built_in_css', true))
-        {
+        if ($this->config->get('plugins.static-social-embeds.built_in_css', true)) {
             $assets->add('plugin://static-social-embeds/assets/css-compiled/sse.min.css', 4);
         }
 
-        if ($this->config->get('plugins.static-social-embeds.built_in_js', true))
-        {
+        if ($this->config->get('plugins.static-social-embeds.built_in_js', true)) {
             $assets->addJs('plugin://static-social-embeds/assets/js/sse.js', 4, true, 'defer');
         }
     }
@@ -86,6 +96,6 @@ public function onAssetsInitialized()
      */
     public function onShortcodeHandlers()
     {
-        $this->grav['shortcode']->registerAllShortcodes(__DIR__.'/shortcodes');
+        $this->grav['shortcode']->registerAllShortcodes(__DIR__ . '/classes/shortcodes');
     }
 }