Allow management of MailTemplateTypes via the admin

[alextreme]

Currently the available mail templates are hardcoded via the settings.

It would be more flexible to move this to the admin and allow administrators to create their own mail template types.

The downside is that it makes testing sending of mail with hardcoded labels more complicated and possibly more error-prone, so it may require a bit more thought (for instance: specifying the minimal available mailtemplates/mailtemplatetypes in the settings and allowing the admin to add additional ones)