Improve handling of HTML tags into Markdown

[StevenJoynt]

add support for HTML tags:

• dl dt dd tags
• code pre samp tt
• dl dt dd
• details summary
• ul ol li dir
• picture (in addition to img tag)

[StevenJoynt]

Investigate using an existing library to convert html to markdown.
It doesn't need to be fast - we're using it in a background job.

Investigate

• https://domchristie.github.io/turndown/ & https://github.com/domchristie/turndown
• https://codebeautify.org/html-to-markdown
• https://github.com/thephpleague/html-to-markdown

[ArchBlood]

@StevenJoynt why not create a convertor similar to the following? Wouldn't this be a better solution than relying on a third-party extension and much easier to maintain?

https://github.com/humhub/humhub/blob/master/protected/humhub/modules/content/widgets/richtext/converter/RichTextToHtmlConverter.php
https://github.com/humhub/humhub/blob/master/protected/humhub/modules/content/widgets/richtext/converter/RichTextToMarkdownConverter.php