Repo avatar fixes (#13891)

- Split up avatar rendering helpers for performance
- Fix showing repo SVG icon when no avatar is set
- Make repo SVG and avatar same size at 32px
- Fix fork line by adding vertical flexbox on repo title

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

web_src/less/_explore.less

@@ -49,11 +49,6 @@
     .ui.tags {
       margin-bottom: 1em;
     }
-
-    .ui.avatar.image {
-      width: 24px;
-      height: 24px;
-    }
   }
 }