[chrstinalin]

Fixes mozilla/addons#16513

Description

Surfaces the unsanitized source of markdown-compatible fields in add-ons through the API, limited to authors.

Testing

See here for allowed Markdown. Anything outside of these is sanitized.

API
GETs from:

1. /api/v5/addons/addon/
2. /api/v5/addons/addon/(int:id|string:slug|string:guid)/
3. /api/v5/addons/addon/(int:addon_id|string:addon_slug|string:addon_guid)/versions/
4. /api/v5/addons/addon/(int:addon_id|string:addon_slug|string:addon_guid)/versions/(int:id|string:version_number)/
5. /api/v5/addons/addon/(int:id|string:slug|string:guid)/eula_policy/

with show_source=true return the Markdown source of HTML fields.

Pinguino
On the listing & details section of an add-on:

1. The description, developer_comments, eula, and privacy_policy fields render the sanitized HTML when not in edit mode.
2. In edit mode, the unsanitized Markdown source is shown.

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 or update relevant docs reflecting the changes made.

[diox]

Update the changelog in the API docs too