Revert Chrome image rendering tweak

The Chrome bug appears to have been fixed:
https://bugs.chromium.org/p/chromium/issues/detail?id=562162

This CSS rule makes images look very ugly in Safari, so removing it.

ref #1259

less/common/Avatar.less

@@ -14,8 +14,6 @@
     height: 100%;
     border-radius: 100%;
     vertical-align: top;
-    // Prevent blurriness in Chrome
-    image-rendering: -webkit-optimize-contrast;
   }
 }