Extract SCSS dropdown z-index variable

app/assets/stylesheets/components/batch_comments/review_bar.scss

@@ -4,7 +4,7 @@
   left: 0;
   width: 100%;
   background: $white;
-  z-index: 300;
+  z-index: $zindex-dropdown-menu;
   padding: 7px 0 6px; // to keep aligned with "collapse sidebar" button on the left sidebar
   border-top: 1px solid $border-color;
   padding-left: $contextual-sidebar-width;

app/assets/stylesheets/framework/awards.scss

@@ -14,7 +14,7 @@
   top: 0;
   margin-top: 3px;
   padding: $gl-padding;
-  z-index: 300;
+  z-index: $zindex-dropdown-menu;
   width: $award-emoji-width;
   font-size: 14px;
   background-color: $white;

app/assets/stylesheets/framework/contextual_sidebar.scss

@@ -471,7 +471,7 @@
       background-color: $black-transparent;
       height: 100%;
       width: 100%;
-      z-index: 300;
+      z-index: $zindex-dropdown-menu;
     }
   }
 }

app/assets/stylesheets/framework/dropdowns.scss

@@ -216,7 +216,7 @@
   position: absolute;
   width: auto;
   top: 100%;
-  z-index: 300;
+  z-index: $zindex-dropdown-menu;
   min-width: 240px;
   max-width: 500px;
   margin-top: $dropdown-vertical-offset;

app/assets/stylesheets/framework/sidebar.scss

@@ -75,7 +75,7 @@
 
 .right-sidebar-expanded {
   padding-right: 0;
-  z-index: 300;
+  z-index: $zindex-dropdown-menu;
 
   @include media-breakpoint-only(sm) {
     &:not(.wiki-sidebar):not(.build-sidebar):not(.issuable-bulk-update-sidebar) .content-wrapper {

app/assets/stylesheets/framework/variables.scss

@@ -433,6 +433,7 @@ $browser-scrollbar-size: 10px;
  */
 $header-height: 40px;
 $header-zindex: 1000;
+$zindex-dropdown-menu: 300;
 $suggestion-header-height: 46px;
 $ide-statusbar-height: 25px;
 $fixed-layout-width: 1280px;

app/assets/stylesheets/pages/merge_requests.scss

@@ -221,7 +221,7 @@ $mr-widget-min-height: 69px;
 
   .mr-widget-pipeline-graph {
     .dropdown-menu {
-      z-index: 300;
+      z-index: $zindex-dropdown-menu;
     }
   }
 

app/assets/stylesheets/utilities.scss

@@ -110,7 +110,7 @@
 
 // This utility is used to force the z-index to match that of dropdown menu's
 .gl-z-dropdown-menu\! {
-  z-index: 300 !important;
+  z-index: $zindex-dropdown-menu !important;
 }
 
 .gl-flex-basis-quarter {