Switch HTML parsing tool from XMLDocument to HTMLAgilityPack's HTMLDocument

[tmehyar]

I'm working on migrating WFFM forms from 8.x to 10.2. When I first ran the tool, most of the Forms failed due to malformed xml/html. For example they had a self-closing img tag without the ending slash <img src="" > or a tag is opened but never closed. These issues are usually understood by a browser but not XMLDocument.

Switching to HTMLDocument in HtmlAgilityPack will solve these issues as it understands these user error like the browser does.
I would create a new HTMLHelper class that uses HTMLDocument and use it wherever we're parsing content that is edited by authors.
I created a fork from this repo and just edited XMLHelper class to use HTMLDocument to overcome these issues.