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/user_cards.tmpl

@@ -8,7 +8,7 @@
 		{{range .Cards}}
 			<li class="item ui segment">
 				<a href="{{.HomeLink}}">
-					<img class="avatar" src="{{.RelAvatarLink}}"/>
+					{{avatar .}}
 				</a>
 				<h3 class="name"><a href="{{.HomeLink}}">{{.DisplayName}}</a></h3>