Fix some migration issues on markdown links.

- Add editorial remarks
- Replace whole markdown to prevent overeager
  replacements (e.g. replacing email addresses
  with mailto links in link text)
Remove accidental regex predicate.
Merge pull request #1041 from alphagov/data-migration-for-markdown-links

Fix some migration issues on markdown links.

https://www.pivotaltracker.com/story/show/60397412