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/shared/issuelist.tmpl

@@ -121,7 +121,7 @@
 				<div class="issue-item-icon-right text grey">
 					{{range .Assignees}}
 						<a class="ui assignee poping up" href="{{.HomeLink}}" data-content="{{.GetDisplayName}}" data-variation="inverted" data-position="left center">
-							<img class="ui avatar image" src="{{.RelAvatarLink}}">
+							{{avatar .}}
 						</a>
 					{{end}}
 				</div>