Provide helpers for replacing links in post content

Most imports involving rich text would benefit from helpers which allow for automatic replacement of urls within body content.

This will likely come in the form of a default WP post iterator which loops through imported articles, looking for links which match the canonical URL of an imported article and replacing it.

Start by tracing how imported articles and rich-text body content are represented, then identify the default WordPress post iterator proposed in the issue. Define how canonical URLs map to imported articles and verify that matching links are replaced without altering unrelated content.

Written by the indexing model from the issue text.