Can't give a rejection reason if previously approved

[topher1kenobe]

A photo was approved, and then we noticed something that required we reject it. We could choose a reason off the drop down list, and the reject button worked, but the text field to write to the submitter was disabled, so we couldn't explain.

[coffee2code]

In 12107:

Photo Directory, Moderation/Rejection: Email contributor of published photo if the photo has subsequently been rejected.

The email is identical to a rejection email but with a line added "Though this photo had already been approved, we have since decided to decline it and have unpublished it from the site.". As such, a required rejection reason and optional mod note to user will be included.

Props topher1kenobe, coffee2code.
Fixes #6365.