Plugin should localise in-line image URLs

[robertandrews]

Describe the bug

When exported, a post containing images/files in the post body show a full, remote image URL still pointing to the original website. This is despite the corresponding images getting downloaded. This defeats some of the purpose and risks a user ending up with no in-line media.

Steps to reproduce the behavior

1. Run export.

Expected behavior

Plugin should localise in-line media URLs so that downloaded ones are used with relative URL paths.
Localised media should not be exclusive to the featured image.

Screenshots