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/header.tmpl

@@ -3,8 +3,8 @@
 	<div class="ui container">
 		<div class="repo-header">
 			<div class="ui huge breadcrumb repo-title">
-				{{if .RelAvatarLink}}
-					<img class="ui avatar image" src="{{.RelAvatarLink}}">
+				{{if .Name}}
+					{{avatar .}}
 				{{else}}
 					{{template "repo/header_icon" .}}
 				{{end}}