[CleanRepo] Add ability to replace redirects from other repos

[gewarren]

Add functionality that replaces site-relative links with their redirect target (if any).

For example:

• Find site-relative links in all .md or .yml files, for example "link text".
• For each different site, prompt the user for the repo path, e.g. "https://github.com/azure-docs".
• Find all the redirection files in the repo, and see if any source_path or source_path_from_root matches the site-relative link in the repo being "cleaned". If so, replace it with the redirect target.

This will require mapping the source_path to its published URL.