Port fix-merge-request-show-component-z-indices to EE

app/assets/stylesheets/framework/sidebar.scss

@@ -53,6 +53,7 @@
 
 .right-sidebar-expanded {
   padding-right: 0;
+  z-index: 300;
 
   @media (min-width: $screen-sm-min) and (max-width: $screen-sm-max) {
     &:not(.wiki-sidebar):not(.build-sidebar) .content-wrapper {

app/assets/stylesheets/pages/issuable.scss

@@ -206,7 +206,7 @@
   transition: width .3s;
   background: $gray-light;
   padding: 10px 20px;
-  z-index: 2;
+  z-index: 200;
 
   &.right-sidebar-expanded {
     width: $gutter_width;

app/assets/stylesheets/pages/merge_requests.scss

@@ -121,6 +121,7 @@
 
     .dropdown-menu {
       margin-top: 11px;
+      z-index: 200;
     }
 
     .ci-action-icon-wrapper {
@@ -696,7 +697,7 @@
   &.affix {
     top: 0;
     left: 0;
-    z-index: 10;
+    z-index: 100;
     transition: right .15s;
 
     @media (max-width: $screen-xs-max) {