Add maintenance pages

app/Http/Middleware/SubmissionsEnabled.php

@@ -0,0 +1,33 @@
+<?php
+
+namespace App\Http\Middleware;
+
+use Closure;
+use Illuminate\Http\Request;
+use Illuminate\Support\Facades\Cache;
+use Symfony\Component\HttpFoundation\Response;
+
+class SubmissionsEnabled
+{
+    /**
+     * Handle an incoming request.
+     *
+     * @param  Closure(Request): (Response)  $next
+     */
+    public function handle(Request $request, Closure $next): Response
+    {
+        if( Cache::get('submissions_maintenance', false ) ){
+            if( $request->expectsJson() ){
+                return response()->json([
+                    'message' => 'The submissions are currently in maintenance mode.',
+                ], 503 );
+            }
+
+            return back()->withErrors([
+                'maintenance' => "The submissions are currently in maintenance mode.",
+            ])->withInput();
+        }
+
+        return $next($request);
+    }
+}