Allow for separate notes to user on approval and rejection

[coffee2code]

When moderating a photo, there exists a field for the moderator to leave a note to the contributor, which gets included in the email sent to the user. Previously, a contributor was only ever sent one email per photo. So if a photo was published then later rejected, the user would not be notified, so there was no need to include a note to the user. However, that has since changed so the contributor is now also emailed if a published photo of theirs gets rejected. Based on our implementation of the note to user field (i.e. a single post meta field), this poses problems:

• The note to user field becomes uneditable once set (since the note already got sent to the user, there is no reason to update it after the fact)
• However, if a note was sent on publication, it would also be sent in the rejection, without the moderator being able to prevent it or change the note.
• Were moderators allowed to update the note, the original note would get overwritten, losing that history.

The site should separately store and display the note sent to the contributor on publication and the note sent to contributor on rejection.

[coffee2code]

In 14055:

Photo Directory, Rejection: Separately store and display notes to user for approval and rejection.

• Allow moderator to leave a note to user when rejecting a published photo
• For pending photos, only show the note to user field corresponding to approval/rejection based on the rejection dropdown choice
• Blank and hide rejection note to user if photo is not being rejected

Fixes #7772.