Test emails are sent (test email templates)

[nlhkabu]

As part of #6598, we have updated the email templates.

I have found that several errors in the templates were not automatically caught by our test suite, including when those errors prevented the email from being sent.

For this particular PR, I will manually test all emails, however, it would be nice to put in place automated tests that catch any email template errors in future.

[ewjoachim]

#9279 may help :) It won't help regarding automated testing, but at least, it will help regarding the ability to manually test the mails.