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/editor/commit_form.tmpl

@@ -1,5 +1,5 @@
 <div class="commit-form-wrapper">
-	<img width="48" height="48" class="ui image commit-avatar" src="{{.SignedUser.RelAvatarLink}}">
+	{{avatar .SignedUser 48 "commit-avatar"}}
 	<div class="commit-form">
 		<h3>{{- if .CanCommitToBranch.WillSign}}
 			<span title="{{.i18n.Tr "repo.signing.will_sign" .CanCommitToBranch.SigningKey}}">{{svg "octicon-lock" 24}}</span>