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

web_src/less/helpers.less

@@ -8,6 +8,7 @@
 .fc { flex-direction: column !important; }
 .f1 { flex: 1 !important; }
 .fw { flex-wrap: wrap !important; }
+.vm { vertical-align: middle !important; }
 
 .mono {
   font-family: var(--fonts-monospace) !important;