Added strikethrough support to back-end md rendering

Needed to tweak the default library strikethrough extension
so that it uses the same element as front-end.
Added testing to cover.
For #2470.

app/Entities/Tools/Markdown/CustomStrikeThroughExtension.php

@@ -0,0 +1,16 @@
+<?php namespace BookStack\Entities\Tools\Markdown;
+
+use League\CommonMark\ConfigurableEnvironmentInterface;
+use League\CommonMark\Extension\ExtensionInterface;
+use League\CommonMark\Extension\Strikethrough\Strikethrough;
+use League\CommonMark\Extension\Strikethrough\StrikethroughDelimiterProcessor;
+
+class CustomStrikeThroughExtension implements ExtensionInterface
+{
+
+    public function register(ConfigurableEnvironmentInterface $environment)
+    {
+        $environment->addDelimiterProcessor(new StrikethroughDelimiterProcessor());
+        $environment->addInlineRenderer(Strikethrough::class, new CustomStrikethroughRenderer());
+    }
+}

app/Entities/Tools/Markdown/CustomStrikethroughRenderer.php

@@ -0,0 +1,24 @@
+<?php namespace BookStack\Entities\Tools\Markdown;
+
+use League\CommonMark\ElementRendererInterface;
+use League\CommonMark\Extension\Strikethrough\Strikethrough;
+use League\CommonMark\HtmlElement;
+use League\CommonMark\Inline\Element\AbstractInline;
+use League\CommonMark\Inline\Renderer\InlineRendererInterface;
+
+/**
+ * This is a somewhat clone of the League\CommonMark\Extension\Strikethrough\StrikethroughRender
+ * class but modified slightly to use <s> HTML tags instead of <del> in order to
+ * match front-end markdown-it rendering.
+ */
+class CustomStrikethroughRenderer implements InlineRendererInterface
+{
+    public function render(AbstractInline $inline, ElementRendererInterface $htmlRenderer)
+    {
+        if (!($inline instanceof Strikethrough)) {
+            throw new \InvalidArgumentException('Incompatible inline type: ' . get_class($inline));
+        }
+
+        return new HtmlElement('s', $inline->getData('attributes', []), $htmlRenderer->renderInlines($inline->children()));
+    }
+}

app/Entities/Tools/PageContent.php

@@ -1,6 +1,7 @@
 <?php namespace BookStack\Entities\Tools;
 
 use BookStack\Entities\Models\Page;
+use BookStack\Entities\Tools\Markdown\CustomStrikeThroughExtension;
 use DOMDocument;
 use DOMNodeList;
 use DOMXPath;
@@ -51,6 +52,7 @@ class PageContent
         $environment = Environment::createCommonMarkEnvironment();
         $environment->addExtension(new TableExtension());
         $environment->addExtension(new TaskListExtension());
+        $environment->addExtension(new CustomStrikeThroughExtension());
         $converter = new CommonMarkConverter([], $environment);
         return $converter->convertToHtml($markdown);
     }