Use polymorphic_url instead of url_for for collaboration mailer

:fork_and_knife: `polymorphic_url` should generate the full URL for `resourceable`. Also add collaborator email mail preview.

According to the [Rails docs](https://github.com/rails/rails/blob/fa0cff484a8f0c99480b7545fc77610009723147/actionpack/lib/action_dispatch/routing/polymorphic_routes.rb#L91) `polymorphic_url` should be the equivalent to calling `cookbook_url(cookbook)` or tool_url(`tool`).