Link to /hosts for admins

This is useful for us to quickly hunt and filter for hostnames in the app.

At the moment, you have to know the URL to get to it.
Remove lib and rake tasks for import mappings

* Redirector is retired. We don't import mappings from it any more.
Merge pull request #405 from alphagov/remove-import-mappings-task

Remove lib and rake tasks for import mappings
Merge pull request #406 from alphagov/show_link_to_hosts_for_admins

Link to /hosts for admins