Changed the values of z-index

Updated app/assets/stylesheets/components/batch_comments/review_bar.scss, app/assets/stylesheets/framework/awards.scss, app/assets/stylesheets/framework/contextual_sidebar.scss, app/assets/stylesheets/framework/dropdowns.scss, app/assets/stylesheets/framework/sidebar.scss, app/assets/stylesheets/pages/merge_requests.scss, app/assets/stylesheets/startup/startup-dark.scss, app/assets/stylesheets/startup/startup-general.scss, app/assets/stylesheets/startup/startup-signin.scss, app/assets/stylesheets/utilities.scss

Still a bit confused what do you mean by you can add the definition right under $header-zindex: 1000; in the variables.scss file 😓

@svedova please let me know if there is any changes that needs to be done in this.