Style blockquote for default issue mail template (#10024)

Style blockquotes as one might expect for html email to disinguish them
from other text

templates/mail/issue/default.tmpl

@@ -5,6 +5,7 @@
 	<title>{{.Subject}}</title>
 
 	<style>
+		blockquote { padding-left: 1em; margin: 1em 0; border-left: 1px solid grey; color: #777}
 		.footer { font-size:small; color:#666;}
 		{{if .ReviewComments}}
 			.review { padding-left: 1em; margin: 1em 0; }