Create a separate notification regarding attachments, similar to comments

Separate the attachment management from the Issue Update event within the notification schemes, allowing users to set up permissions relative to adding and deleting attachments - similar to how you have broken out comment management.

We can get spammed with email when a user adds one attachment when using Safari, and it adds all the other images as attachments - sometimes creating 15 random attachments. Consequently, since there isn't a mass attachment deletion feature, we have to delete each one individual. And for each deletion, it sends out separate notifications.