mod reports: enable reporting for members, add dmail reporting.

* Add ability to report dmails.
* Enable reports for comments, forum posts, and dmails.
* Allow Members to send reports.
* Don't allow users to report the same thing twice.

app/views/moderation_reports/create.js.erb

@@ -1 +1 @@
-Danbooru.notice("Report submitted.");
+Danbooru.notice("<%= j flash[:notice] %>");