Refactor collapsed comments' @extend usage

Just some small adjustments to how @extend is used here, so that some of
the styles that apply to "fully collapsed" chains don't get brought over
to the "individually collapsed" comments when they're not wanted.

The % syntax is SASS's recommendation for "@extend-only selectors".

tildes/scss/_themes.scss

@@ -116,7 +116,7 @@
     .label-comment-tag-flame { @include specialtag($comment-tag-flame-color, $is-light); }
   }
 
-  .is-comment-collapsed {
+  %collapsed {
     header {
       background-color: $background-color;
       color: $text-secondary-color;
@@ -127,6 +127,16 @@
     }
   }
 
+  .is-comment-collapsed {
+    @extend %collapsed;
+  }
+
+  .is-comment-collapsed-individual {
+    & > .comment-itself {
+      @extend %collapsed;
+    }
+  }
+
   .is-comment-deleted, .is-comment-removed {
     color: $text-secondary-color;
   }

tildes/scss/modules/_comment.scss

@@ -127,11 +127,7 @@
   }
 }
 
-.is-comment-collapsed {
-  &[data-comment-depth="0"] {
-    border-bottom: 0;
-  }
-
+%collapsed {
   .comment-edited-time,
   .comment-nav-link,
   .comment-posted-time,
@@ -143,10 +139,6 @@
     display: none;
   }
 
-  .comment-branch-counter {
-    display: inline-block;
-  }
-
   .comment-excerpt {
     display: inline-block;
   }
@@ -156,11 +148,23 @@
   }
 }
 
+.is-comment-collapsed {
+  @extend %collapsed;
+
+  &[data-comment-depth="0"] {
+    border-bottom: 0;
+  }
+
+  .comment-branch-counter {
+    display: inline-block;
+  }
+}
+
 // uses @extend to only collapse everything inside the collapsed comment itself and
 // not its replies
 .is-comment-collapsed-individual {
   & > .comment-itself {
-    @extend .is-comment-collapsed;
+    @extend %collapsed;
   }
 }