Comments: Added HTML filter on load, tinymce elem filtering

- Added filter on load to help prevent potentially dangerous comment
  HTML in DB at load time (if it gets passed input filtering, or is
  existing).
- Added TinyMCE valid_elements for input wysiwygs, to gracefully degrade
  content at point of user-view, rather than surprising the user by
  stripping content, which TinyMCE would show, post-save.

app/Activity/Models/Comment.php

@@ -4,6 +4,7 @@ namespace BookStack\Activity\Models;
 
 use BookStack\App\Model;
 use BookStack\Users\Models\HasCreatorAndUpdater;
+use BookStack\Util\HtmlContentFilter;
 use Illuminate\Database\Eloquent\Factories\HasFactory;
 use Illuminate\Database\Eloquent\Relations\BelongsTo;
 use Illuminate\Database\Eloquent\Relations\MorphTo;
@@ -73,4 +74,9 @@ class Comment extends Model implements Loggable
     {
         return "Comment #{$this->local_id} (ID: {$this->id}) for {$this->entity_type} (ID: {$this->entity_id})";
     }
+
+    public function safeHtml(): string
+    {
+        return HtmlContentFilter::removeScriptsFromHtmlString($this->html ?? '');
+    }
 }