Input WYSIWYG: Added description_html field, added store logic

Rolled out HTML editor field and store logic across all target entity
types. Cleaned up WYSIWYG input logic and design.
Cleaned up some injected classes while there.

app/Entities/Models/HasHtmlDescription.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace BookStack\Entities\Models;
+
+use BookStack\Util\HtmlContentFilter;
+
+/**
+ * @property string $description
+ * @property string $description_html
+ */
+trait HasHtmlDescription
+{
+    /**
+     * Get the HTML description for this book.
+     */
+    public function descriptionHtml(): string
+    {
+        $html = $this->description_html ?: '<p>' . e($this->description) . '</p>';
+        return HtmlContentFilter::removeScriptsFromHtmlString($html);
+    }
+}