templates: use owner's avatar as image of repository

templates/base/head.tmpl

@@ -29,6 +29,7 @@
 		<meta property="og:type" content="object" />
 		<meta property="og:title" content="{{.Repository.FullName}}">
 		<meta property="og:description" content="{{.Repository.Description}}">
+		<meta property="og:image" content="{{.Repository.Owner.AvatarLink}}" />
 	{{else}}
 		<meta property="og:url" content="{{AppUrl}}" />
 		<meta property="og:type" content="website" />