Google Code-in

This issue is a part of project #3331

Create a new mailer as following:

1. First, the mailer would send the email to comment author notifying that comment has been approved.
2. Create a view of email by creating a new file in admin_mailer
3. Write functional tests to check whether flow is working as planned or not. Name the method as notify_author_of_comment_approval and also use the same name for the file.

Refer the method https://github.com/publiclab/plots2/blob/master/app/mailers/admin_mailer.rb#L19-L26