Automated emails reverting to standard concepts

[GitBreax]

Expected behaviour

Sending a mail to participants of an activity.

Actual behavior

It throws out the standard payment email instead of the writen one.

Steps to reproduce

• admin/user Log in as admin, go to activity, write email, press 'verstuur'
• browser: https://koala.svsticky.nl/activities/####

[Riscky]

This should be fixed with #977