Long-form feedback: separate user-specified link

Users can provide a URL when leaving long-form feedback. This shouldn't
be treated in the same way as an automatically-determined URL, because
the user-specified link could contain typos or not be a link at all.
This change starts storing this information in a separate field.
Fix the URL on long-form feedbacks
Merge pull request #142 from alphagov/separate-user-specified-and-originating-url-on-long-form-feedback

Separate user-specified and originating url on long form feedback