improved invoices with new renderer (#306)

* added docx renderer and demo template
* added csv renderer and demo template
* added xlsx renderer and demo template
* added ods renderer and demo template
* added user calculator
* added invoice documentation

src/Invoice/NumberGenerator/DateNumberGenerator.php

@@ -0,0 +1,49 @@
+<?php
+
+/*
+ * This file is part of the Kimai time-tracking app.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace App\Invoice\NumberGenerator;
+
+use App\Invoice\NumberGeneratorInterface;
+use App\Model\InvoiceModel;
+
+/**
+ * Class DateNumberGenerator generates the invoice number based on the current day.
+ * It will create duplicate IDs if you create multiple invoices per day.
+ */
+class DateNumberGenerator implements NumberGeneratorInterface
+{
+    /**
+     * @var InvoiceModel
+     */
+    protected $model;
+
+    /**
+     * @return string
+     */
+    public function getId(): string
+    {
+        return 'default';
+    }
+
+    /**
+     * @param InvoiceModel $model
+     */
+    public function setModel(InvoiceModel $model)
+    {
+        $this->model = $model;
+    }
+
+    /**
+     * @return string
+     */
+    public function getInvoiceNumber(): string
+    {
+        return date('ymd', $this->model->getInvoiceDate()->getTimestamp());
+    }
+}