Support Markdown in Add-on Listing Fields

[chrstinalin]

Fixes: mozilla/addons#15145

Description

Replaces HTML syntax support with Markdown support in the description, developer_comments, eula, and privacy_policy fields of Addon. Existing HTML in those fields will remain functional until the field is edited. No new HTML will be functional.

Only the markdown-equivalent of currently allowed HTML attributes are allowed. abbr uses the PHP Markdown Extra syntax.

DevHub:

Frontend:

Testing

Checklist

• Add #ISSUENUM at the top of your PR to an existing open issue in the mozilla/addons repository.
• Successfully verified the change locally.
• The change is covered by automated tests, or otherwise indicated why doing so is unnecessary/impossible.
• Add before and after screenshots (Only for changes that impact the UI).
• Add or update relevant docs reflecting the changes made.