Edit notification texts - wrong placeholders

This bug still persists into the 7.x-1.9 version

@bfodeke, I'm pretty sure you can use work hours at Classic to patch this.

There's a mixture of @variables and %variables used throughout the module and also in the stored messages, which may have been customized by the admin. I believe it's best to just support both variable types.

Here's a patch that does this.