Add API (#606)

This is the first part of the API. It supports most of contacts, as well as the
journal. This PR also brings Monica to Laravel 5.5, as the API required it with
the new concept of resources. It needs to run `php artisan migrate` as well as
`php artisan passport:keys` to generate the access tokens needed for the API.

app/Http/Resources/Debt/DebtCollection.php

@@ -0,0 +1,24 @@
+<?php
+
+namespace App\Http\Resources\Debt;
+
+use Illuminate\Http\Resources\Json\ResourceCollection;
+
+class DebtCollection extends ResourceCollection
+{
+    /**
+     * Transform the resource collection into an array.
+     *
+     * @param  \Illuminate\Http\Request
+     * @return array
+     */
+    public function toArray($request)
+    {
+        return [
+            'data' => $this->collection,
+            'links' => [
+                'self' => 'link-value',
+            ],
+        ];
+    }
+}