#33 send email to bad_posters

This deals with https://github.com/maiera/gde-app/issues/33

We just need to agree on the template & send emails to the post owners instead of me.
If we want to test if the elements of the lists are actually the same, and not only the same amount, e.g. if someone realizes he has mis-tagged a post with the wrong product group, we could also use this for comparison:
`if sorted(entity.product_group) != sorted(prod_group)`