Use url xor color on avatar, never both

Combining them often leaves a crufty colored ring on the outer edge of
the avatar image.

// FREEBIE

background.html

@@ -139,7 +139,12 @@
   </script>
   <script type='text/x-tmpl-mustache' id='avatar'>
     <span class='avatar'
-          style='background-image: url("{{ avatar.url }}"); background-color: {{ avatar.color }}'>
+          {{ #avatar.url }}
+            style='background-image: url("{{ avatar.url }}");'>
+          {{ /avatar.url }}
+          {{ ^avatar.url }}
+            style='background-color: {{ avatar.color }};'>
+          {{ /avatar.url }}
         {{ avatar.content }}
     </span>
   </script>

stylesheets/_global.scss

@@ -209,7 +209,6 @@ $avatar-size: 44px;
   display: inline-block;
   height: $avatar-size;
   width: $avatar-size;
-  background: $grey_l url('/images/default.png') no-repeat center;
   border-radius: 50%;
   background-size: cover;
   vertical-align: middle;

stylesheets/manifest.css

@@ -173,7 +173,6 @@ img.emoji {
   display: inline-block;
   height: 44px;
   width: 44px;
-  background: #f3f3f3 url("/images/default.png") no-repeat center;
   border-radius: 50%;
   background-size: cover;
   vertical-align: middle;