move .gap width into desktop styles

app/assets/stylesheets/common/base/topic-post.scss

@@ -204,7 +204,6 @@ blockquote > *:last-child {
   color: lighten($primary, 70%);
   cursor: pointer;
   text-align: center;
-  width: calc(#{$topic-avatar-width} + #{$topic-body-width} + 2 * #{$topic-body-width-padding});
 
   &.jagged-border {
     background-image:

app/assets/stylesheets/desktop/topic-post.scss

@@ -709,6 +709,10 @@ $topic-avatar-width: 45px;
   z-index: 2;
 }
 
+.gap {
+  width: calc(#{$topic-avatar-width} + #{$topic-body-width} + 2 * #{$topic-body-width-padding});
+}
+
 .time-gap {
   width: 755px;
   border-top: 1px solid dark-light-diff($primary, $secondary, 90%, -60%);