Moved right-sidebar styles out of the left column

In order to properly refactor the left-hand panel we should be
careful with the shared styles

app/assets/stylesheets/framework/contextual_sidebar.scss

@@ -363,21 +363,8 @@
 // Collapsed nav
 
 .toggle-sidebar-button,
-.close-nav-button,
-.toggle-right-sidebar-button {
-  transition: width $sidebar-transition-duration;
-  height: $toggle-sidebar-height;
-  padding: 0 $gl-padding;
-  background-color: $gray-light;
-  border: 0;
-  color: $gl-text-color-secondary;
-  display: flex;
-  align-items: center;
-
-  &:hover {
-    background-color: $border-color;
-    color: $gl-text-color;
-  }
+.close-nav-button {
+  @include side-panel-toggle;
 }
 
 .toggle-sidebar-button,
@@ -396,10 +383,6 @@
   }
 }
 
-.toggle-right-sidebar-button {
-  border-bottom: 1px solid $border-color;
-}
-
 .collapse-text {
   white-space: nowrap;
   overflow: hidden;

app/assets/stylesheets/framework/mixins.scss

@@ -446,3 +446,19 @@
     }
   }
 }
+
+@mixin side-panel-toggle {
+  transition: width $sidebar-transition-duration;
+  height: $toggle-sidebar-height;
+  padding: 0 $gl-padding;
+  background-color: $gray-light;
+  border: 0;
+  color: $gl-text-color-secondary;
+  display: flex;
+  align-items: center;
+
+  &:hover {
+    background-color: $border-color;
+    color: $gl-text-color;
+  }
+}

app/assets/stylesheets/framework/sidebar.scss

@@ -232,3 +232,8 @@
     }
   }
 }
+
+.toggle-right-sidebar-button {
+  @include side-panel-toggle;
+  border-bottom: 1px solid $border-color;
+}