Added conversion of iframes to anchors on PDF export

- Replaced iframe elements with anchor elements wrapped in a paragraph.
- Extracted PDF generation action to seperate class for easier mocking
  within testing.
- Added test to cover.

For #3077

app/Entities/Tools/PdfGenerator.php

@@ -0,0 +1,28 @@
+<?php
+
+namespace BookStack\Entities\Tools;
+
+use Barryvdh\Snappy\Facades\SnappyPdf;
+use Barryvdh\DomPDF\Facade as DomPDF;
+
+class PdfGenerator
+{
+
+    /**
+     * Generate PDF content from the given HTML content.
+     */
+    public function fromHtml(string $html): string
+    {
+        $useWKHTML = config('snappy.pdf.binary') !== false && config('app.allow_untrusted_server_fetching') === true;
+
+        if ($useWKHTML) {
+            $pdf = SnappyPDF::loadHTML($html);
+            $pdf->setOption('print-media-type', true);
+        } else {
+            $pdf = DomPDF::loadHTML($html);
+        }
+
+        return $pdf->output();
+    }
+
+}