Comments: Notification after comment is approved

#### Current behavior in comments
- A user writes a comment and is held for moderation (when this option is on);
- The comment is approved and published;
- The user does not know if the comment was published because he did not receive notification, he needs to go to the site to check it out.

#### Solution I would like
I would like the user to be able to choose an option to allow receiving an email notification when the comment is approved.

#### Alternative that I considered
The Wpdiscuz plugin is an alternative that has the desired functionality.

The issue names no files, tests, or entry points. Begin by locating the comment moderation flow and any existing notification settings, then define the approval-notification behavior and verify both opt-in and opt-out cases.

Written by the indexing model from the issue text.