When you comment an issue, make it optional to receive an email Notification (BB-6173)

I'm a fluent commenter ... and since the new redesign I'm looking into replacing my current issue system with bitbucket.

So to make it a better experience.

1. If you are adding a comment from the web, you are already "notified", so make it optional to receive an email notification.
2. If the email notifications are disabled, however you are been mention on the comment, send the email notification anyways. Or better add a new option "Receive email notifications only if you are mentioned"