Added user setting system and added user-lang option

Supports #115

app/Http/Middleware/Localization.php

@@ -0,0 +1,25 @@
+<?php
+
+namespace BookStack\Http\Middleware;
+
+use Carbon\Carbon;
+use Closure;
+
+class Localization
+{
+    /**
+     * Handle an incoming request.
+     *
+     * @param  \Illuminate\Http\Request  $request
+     * @param  \Closure  $next
+     * @return mixed
+     */
+    public function handle($request, Closure $next)
+    {
+        $defaultLang = config('app.locale');
+        $locale = setting()->getUser(user(), 'language', $defaultLang);
+        app()->setLocale($locale);
+        Carbon::setLocale($locale);
+        return $next($request);
+    }
+}