Use mixin for new dropdown style

app/assets/stylesheets/framework/dropdowns.scss

@@ -725,7 +725,8 @@
 
 // TODO: change global style and remove mixin
 @mixin new-style-dropdown {
-  .dropdown-menu {
+  .dropdown-menu,
+  .dropdown-menu-nav {
     li {
       padding: 0 1px;
 
@@ -766,4 +767,8 @@
       }
     }
   }
+
+  .dropdown-menu-align-right {
+    margin-top: 2px;
+  }
 }

app/assets/stylesheets/framework/header.scss

@@ -4,6 +4,8 @@
  */
 
 header {
+  @include new-style-dropdown;
+
   transition: padding $sidebar-transition-duration;
 
   &.navbar-empty {
@@ -313,25 +315,6 @@ header {
   .impersonation i {
     color: $red-500;
   }
-
-  // TODO: fallback to global style
-  .dropdown-menu,
-  .dropdown-menu-nav {
-    li {
-      padding: 0 1px;
-
-      a {
-        border-radius: 0;
-        padding: 8px 16px;
-
-        &:hover,
-        &:active,
-        &:focus {
-          background-color: $gray-darker;
-        }
-      }
-    }
-  }
 }
 
 .with-performance-bar header.navbar-gitlab {

app/assets/stylesheets/pages/cycle_analytics.scss

 #cycle-analytics {
+  @include new-style-dropdown;
+
   max-width: 1000px;
   margin: 24px auto 0;
   position: relative;
@@ -110,10 +112,6 @@
 
   .js-ca-dropdown {
     top: $gl-padding-top;
-
-    .dropdown-menu-align-right {
-      margin-top: 2px;
-    }
   }
 
   .content-list {
@@ -446,24 +444,6 @@
       margin-bottom: 20px;
     }
   }
-
-  // TODO: fallback to global style
-  .dropdown-menu {
-    li {
-      padding: 0 1px;
-
-      a {
-        border-radius: 0;
-        padding: 8px 16px;
-
-        &:hover,
-        &:active,
-        &:focus {
-          background-color: $gray-darker;
-        }
-      }
-    }
-  }
 }
 
 .cycle-analytics-overview {

app/assets/stylesheets/pages/tree.scss

 .tree-holder {
+  @include new-style-dropdown;
 
   .nav-block {
     margin: 10px 0;
@@ -202,28 +203,6 @@
       }
     }
   }
-
-  // TODO: fallback to global style
-  .dropdown-menu:not(.dropdown-menu-selectable) {
-    li {
-      padding: 0 1px;
-
-      &.dropdown-header {
-        padding: 8px 16px;
-      }
-
-      a {
-        border-radius: 0;
-        padding: 8px 16px;
-
-        &:hover,
-        &:active,
-        &:focus {
-          background-color: $gray-darker;
-        }
-      }
-    }
-  }
 }
 
 .blob-commit-info {