Add Internal Comments for Testimonials

It would be amazing to have a custom field for internal use to write any comments we want for each testimonial. This will give flexibility to create a lot of various workflows for us.

For me personally, I would like to write when was the last time I used the testimonial in various socials and newsletter. I want to avoid redundancy and send the same testimonial too often. I want to keep it fresh and go through a lot of testimonials one at the time.