Unexpected preview path for images outside of media_library

[erquhart]

Describe the bug

See #3240 for context. That issue is the final fix for this, this issue is toward graceful failure.

• If a collection of markdown documents have images from multiple directories in their document body, the CMS can only properly handle one of those directories.
• Images from outside that directory are shown in the CMS with src set to the media_folder, except in the case of an absolute media_folder, the opening slash is removed, resulting in a relative path.

Example scenario:

• combined media_folder for collection:
  /static/Images/docbook/images/api_mgmt
• markdown image tag:
  ![API Gateway features](/Images/docbook/images/concepts/api_server.png)
• resulting HTML img tag src in the CMS editor:
  static/Images/docbook/images/api_mgmt/api_server.png

To Reproduce

An entry in a deploy preview for a site implementing Open Authoring shows this behavior:
https://deploy-preview-203--axway-open-docs.netlify.com/admin/#/collections/apimgr_concepts/entries/apigtw_introduction

The Axway open docs site can be forked and run locally for testing:
https://github.com/Axway/axway-open-docs

Expected behavior

• resulting HTML img tag src in the CMS editor should fall back to the URL provided in the content:
  /Images/docbook/images/concepts/api_server.png