Ability to customize the email notifications from admin panel

[ahukkanen]

We have installed this module for few customers so far and it seems that everyone wants to modify the email messages to fit to their needs.

I suggest making it possible to modify the messages per organization from the admin panel, similarly as we can currently modify e.g. the welcome notification for new user accounts. The default messages can come from the locales as defined currently but if the custom message option is enabled, it would use the custom message instead.