Provide user-facing rationale to show in "your report was moderated" email.

[neprune]

When a report is moderated, the reporter currently receives an email informing them of the changes made, or if the report was hidden, but does not include any rationale for why the change was made. This can be frustrating for users and is also potentially a missed opportunity to prevent them making future reports that require moderation.

An optional field could be added to the moderation form to allow the moderator to provide a message that is included in moderation email. It could look like this: