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

1. Navigate to existing add-on that already has HTML in its description. The HTML should still be functional. The field is still formatted on the frontend listing.
2. Under the 'edit' page, the description and developer comments should support markdown syntax.*
3. Under the 'Manage Authors & License' page, the EULA and Privacy Policy fields should support markdown syntax.*

* Any user-inputted HTML saved at this point no longer works. Any markdown in the field is formatted in the frontend, and HTML is not.

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.

[chrstinalin]

There should probably be a more informative way to explain what syntax or Markdown is supported, possibly similar to how GitHub explains it via the 'Markdown is Supported' button by navigating to a dedicated page.

[KevinMind]

Looks promising. A few comments before testing.

[KevinMind]

allowed_markdown should be in a constant somewhere. If these are in fact the only tags allowed then use that same constant in actual processing.

[chrstinalin]

Just since you pointed this out --

There should probably be a more informative way to explain what syntax or Markdown is supported, possibly similar to how GitHub explains it via the 'Markdown is Supported' button by navigating to a dedicated page.

I mentioned this to Andrew offhandedly as, especially with the extensions, it may not be obvious what Markdown syntax is allowed in what way, and he mentioned maybe making it a page on the extension workshop over the tooltip. What do you think?

[KevinMind]

Please add testing steps and I can verify the PR.