Add rake task for links

Adds a rake task that can be run as `rake links:check` that checks all the links it can find in the SA yaml files.

Creates `log/broken_links.log` (all links that either 3XX or 4XX) and `log/404_links.log`, which contains links that 404.

Useful for finding broken links (currently got around 25, but https://github.com/alphagov/smart-answers/pull/331 brings that number down substantially).