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/issue/view_content/pull.tmpl

@@ -9,8 +9,8 @@
 					<div class="review-item">
 						<div class="review-item-left">
 							{{if .User}}
-								<a class="ui avatar image" href="{{.User.HomeLink}}">
-									<img src="{{.User.RelAvatarLink}}">
+								<a href="{{.User.HomeLink}}">
+									{{avatar .User}}
 								</a>
 							{{end}}
 							<span class="text grey">