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

tests/Entity/ExportTest.php

@@ -6,6 +6,7 @@ use BookStack\Auth\Role;
 use BookStack\Entities\Models\Book;
 use BookStack\Entities\Models\Chapter;
 use BookStack\Entities\Models\Page;
+use BookStack\Entities\Tools\PdfGenerator;
 use Illuminate\Support\Facades\Storage;
 use Illuminate\Support\Str;
 use Tests\TestCase;
@@ -289,6 +290,24 @@ class ExportTest extends TestCase
         $resp->assertDontSee('ExportWizardTheFifth');
     }
 
+    public function test_page_pdf_export_converts_iframes_to_links()
+    {
+        $page = Page::query()->first()->forceFill([
+            'html'     => '<iframe width="560" height="315" src="//www.youtube.com/embed/ShqUjt33uOs"></iframe>',
+        ]);
+        $page->save();
+
+        $pdfHtml = '';
+        $mockPdfGenerator = $this->mock(PdfGenerator::class);
+        $mockPdfGenerator->shouldReceive('fromHtml')
+            ->with(\Mockery::capture($pdfHtml))
+            ->andReturn('');
+
+        $this->asEditor()->get($page->getUrl('/export/pdf'));
+        $this->assertStringNotContainsString('iframe>', $pdfHtml);
+        $this->assertStringContainsString('<p><a href="https://www.youtube.com/embed/ShqUjt33uOs">https://www.youtube.com/embed/ShqUjt33uOs</a></p>', $pdfHtml);
+    }
+
     public function test_page_markdown_export()
     {
         $page = Page::query()->first();