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_icon.tmpl

@@ -7,7 +7,7 @@
 		{{else if and (not $.IsMirror) (not $.IsFork) ($.Owner)}}
 			{{svg "octicon-repo" 32}}
 			{{if $.Owner.Visibility.IsPrivate}}
-				<img class="ui avatar image" src="{{$.Owner.RelAvatarLink}}">
+				{{avatar $.Owner}}
 			{{end}}
 		{{else if $.IsMirror}}
 			{{svg "octicon-mirror" 32}}