Refactor and reword emails.

Emails were sent in html but didn't include links. And they didn't really look that pretty. This uses a [simple responsive email template](https://github.com/leemunroe/html-email-template) as a basis, with [premailer-rails](https://github.com/fphilipe/premailer-rails) to convert the html to a text-part as well (for those mutt users). I took the liberty to reword the texts too, but hope to find someone to review it. Still, I think it's an improvement already, and worthwhile to share. (And it keeps the differences between my fork and yours small.)