Direct avatar rendering (#13649)

* Direct avatar rendering

This adds new template helpers for avatar rendering which output image
elements with direct links to avatars which makes them cacheable by the
browsers.

This should be a major performance improvment for pages with many avatars.

* fix avatars of other user's profile pages

* fix top border on user avatar name

* uncircle avatars

* remove old incomplete avatar selector

* use title attribute for name and add it back on blame

* minor refactor

* tweak comments

* fix url path join and adjust test to new result

* dedupe functions

templates/repo/forks.tmpl

@@ -8,7 +8,7 @@
 		<div class="ui list">
 			{{range .Forks}}
 				<div class="item">
-					<img class="ui avatar image" src="{{.Owner.RelAvatarLink}}">
+					{{avatar .Owner}}
 					<div class="link">
 						<a href="{{AppSubUrl}}/{{.Owner.Name}}">{{.Owner.Name}}</a>
 						/