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/_user.less

@@ -15,6 +15,10 @@
         line-height: 1.3rem;
       }
 
+      .profile-avatar-name {
+        border-top: none;
+      }
+
       .extra.content {
         padding: 0;
 
@@ -46,9 +50,16 @@
       }
 
       #profile-avatar {
+        background: none;
+        padding: 1rem 1rem .25rem;
+
         img {
           width: 100%;
+          height: auto;
+          object-fit: contain;
+          margin: 0;
         }
+
         @media @mediaSm {
           height: 250px;
           overflow: hidden;