FEATURE #1373: add history of content

This commit contains only core funcionality of displaying history. Module settings must be implemented in each module.

app/Handlers/Contracts/AppHandler.php

@@ -0,0 +1,30 @@
+<?php
+namespace App\Handlers\Contracts;
+
+use App\Models\Content;
+use App\Models\User;
+
+interface AppHandler
+{
+    /**
+     * Render given module into visitor area.
+     *
+     * @param Content $content    DB model from table contents
+     * @param array   $parameters parameters used for modifications/custom settings of given model
+     *
+     * @return mixed
+     */
+    public function render(Content $content, $parameters);
+
+    public static function getHistoryHandler();
+
+    /**
+     * Register admin menu (or multiple ones)
+     *
+     * This cannot be in ServiceProvider because Session is started at middleware. And ServiceProvider is called before
+     * starting any middleware..
+     *
+     * @param User $user model of logged user
+     */
+    public static function registerAdminMenu(User $user);
+}