Better support for internal review requests being sent by mail

[crowbot]

• [ ] Additions to text on the ipvtheme page for requesting an internal review - https://github.com/mysociety/ipvtheme/issues/2
• [ ] A custom action to download the text of a request for internal review, ready for printing https://github.com/mysociety/ipvtheme/issues/3
• [ ] An email alert to be sent to the requester 30 days after a request for internal review has been made https://github.com/mysociety/ipvtheme/issues/4
• [ ] Change link in followup template to point to help section on requesting https://github.com/mysociety/ipvtheme/issues/5
• [ ] Markup internal review text for translation

[crowbot]

The marking up of the internal review request is in https://github.com/mysociety/alaveteli/pull/930