Automatic export of GitLab terms as docx

Right now, if somebody wants to make a change to the GitLab terms, they need to export them to PDF from our website, copy-paste them to a docx file, and then take time to manually format the text.

We might be able to make pandoc do this for us. Pandoc supports docx: http://johnmacfarlane.net/pandoc/

According to @jacobvosmaer , following steps are needed to create a link on our website next to the PDF preview link:

1. get a working pandoc command that turns a file like public/terms/print/gitlab_subscription_terms.html into public/terms/print/gitlab_subscription_terms.docx

2. add a rake task to build docx files in the Rakefile near https://gitlab.com/gitlab-com/www-gitlab-com/blob/410e679604370d92494735b486987efe71bf60af/Rakefile#L406

3. add links to the docx files in the terms index.html, see https://gitlab.com/gitlab-com/www-gitlab-com/blob/410e679604370d92494735b486987efe71bf60af/source/terms/index.html#L21

/cc @sytses @Haydn