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/explore/organizations.tmpl

@@ -7,7 +7,7 @@
 		<div class="ui user list">
 			{{range .Users}}
 				<div class="item">
-				  <img class="ui avatar image" src="{{.RelAvatarLink}}">
+					{{avatar .}}
 				  <div class="content">
 					<span class="header">
 						<a href="{{.HomeLink}}">{{.Name}}</a> {{.FullName}}

templates/explore/repo_list.tmpl

@@ -2,9 +2,7 @@
 	{{range .Repos}}
 		<div class="item">
 			<div class="ui header">
-				{{if .RelAvatarLink}}
-					<img class="ui avatar image" src="{{.RelAvatarLink}}">
-				{{end}}
+				{{avatar .}}
 				<a class="name" href="{{.Link}}">
 					{{if or $.PageIsExplore $.PageIsProfileStarList }}{{if .Owner}}{{.Owner.Name}} / {{end}}{{end}}{{.Name}}
 				</a>

templates/explore/users.tmpl

@@ -7,7 +7,7 @@
 		<div class="ui user list">
 			{{range .Users}}
 				<div class="item">
-				  <img class="ui avatar image" src="{{.RelAvatarLink}}">
+					{{avatar .}}
 				  <div class="content">
 					<span class="header"><a href="{{.HomeLink}}">{{.Name}}</a> {{.FullName}}</span>
 					<div class="description">