Validate some common errors in links.

Show error messages when editors put in non-absolute links, hover text, or rel=external in their markdown.

https://www.agileplannerapp.com/boards/173808/cards/3357
This regex is somewhat inscrutable. Is it worth expanding and commenting it using the `x` flag (eg https://github.com/alphagov/panopticon/blob/master/config/routes.rb#L5-L13)?