CONN-130: Allow HTML5 elements in ArticleComment html

This function is called in ArticleComment and it's purpose is to normalize the HTML content from the parser. Thumbnails are embedded using the `<figure>` HTML5 element, which is invalidated if the DOCTYPE is not set in the HTML text.

The change adds the proper HTML5 `<!DOCTYPE html>` for the HTML string and further fixes some small code formatting issues.

Test case is also added to the Unit test, to validate the change.