Add HTML abuse report notification email.

app/views/abuse_report_mailer/notify.html.haml

+%p
+  #{link_to @abuse_report.user.name, user_url(@abuse_report.user)} 
+  (@#{@abuse_report.user.username}) was reported for abuse by 
+  #{link_to @abuse_report.reporter.name, user_url(@abuse_report.reporter)} 
+  (@#{@abuse_report.reporter.username}).
+
+%blockquote
+  = @abuse_report.message
+
+%p
+  = link_to "View details", abuse_reports_url