Fix border radius of collapse discussion btn

Turn avatar-counter into a mixin with optional border-radius

app/assets/stylesheets/components/avatar.scss

@@ -191,12 +191,5 @@ $identicon-backgrounds: $identicon-red, $identicon-purple, $identicon-indigo, $i
 }
 
 .avatar-counter {
-  background-color: $gray-darkest;
-  color: $white-light;
-  border: 1px solid $gray-normal;
-  border-radius: 1em;
-  font-family: $regular-font;
-  font-size: 9px;
-  line-height: 16px;
-  text-align: center;
+  @include avatar-counter();
 }

app/assets/stylesheets/framework/mixins.scss

@@ -396,3 +396,14 @@
   width: $gl-font-size * $code-line-height * 0.9;
   height: $gl-font-size * $code-line-height * 0.9;
 }
+
+@mixin avatar-counter($border-radius: 1em) {
+  background-color: $gray-darkest;
+  color: $white-light;
+  border: 1px solid $gray-normal;
+  border-radius: $border-radius;
+  font-family: $regular-font;
+  font-size: 9px;
+  line-height: 16px;
+  text-align: center;
+}

app/assets/stylesheets/pages/diff.scss

@@ -676,7 +676,7 @@ table.code {
 
 .diff-comments-more-count,
 .diff-notes-collapse {
-  @extend .avatar-counter;
+  @include avatar-counter(50%);
 }
 
 .diff-notes-collapse {