refactor templates with mailto: links

Created by: das-g

In #373 (closed) I touched two partials that create mailto: links with subject and body. The creation of those links would benefit from being extracted to a separate, new partial:

• The templates currently creating those links semi-hardcoded would become cleaner
• DRY
• It would force us to move the generation of the link content (which will be the context of the new partial) to python, leading to better separation. (Python might in turn generate the content by rendering yet another template, if suitable. That might then be reused in #371)