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/diff/comments.tmpl

@@ -6,7 +6,7 @@
 		<span class="avatar"><img src="/img/avatar_default.png"></span>
 	{{else}}
 		<a class="avatar" {{if gt .Poster.ID 0}}href="{{.Poster.HomeLink}}"{{end}}>
-			<img src="{{.Poster.RelAvatarLink}}">
+			{{avatar .Poster}}
 		</a>
 	{{end}}
 	<div class="content comment-container">