851 review apps landing page

Closes #851

Meta Issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/20255

• Release: 8.14
• Due date: Nov 18th
• Merge: Nov 22th

URL: /features/review-apps/

Tasks:

• New hero section (the icon should be review icon from the 10 steps and the background screen should be updated)

• Add the collaboration icon that David shared and the below after automatic live preview

• Benefit: One-click to collaborate

• Supporting text: No need to check out a branch or spin up a staging environment. Just send a link and let your team click around.

• Remove the Review Apps overview section. Since the screenshot does not add value and it takes up a decent amount of space.

• Remove the icons, screens, and copy for all of the benefits

• In the place of the benefits, let's use this portion of Dimitrie's image

• The full image was shared here: !3750 (comment 18497551)

• Above the image it should say: "Review Apps transform the way you and your team work. For developers this means you can kick off your feedback process much sooner. Submit a merge request and preview your changes in a live environment. When you push to master, you'll know your changes have gone through CI testing as well as a live review where the team can make sure things actually work.

• For designers, product managers, marketing counterparts, etc., giving feedback just got a lot easier. You won't have to check out branches or spin up a staging environment to preview changes. With Review Apps, you click a link and see the live changes. Hopefully, this means you can give more timely and informed feedback."

• Make size of buttons under Get Started with GitLab Review Apps section be equal.

source/features/review-apps/index.html.haml

@@ -67,10 +67,15 @@ extra_js:
 .page-container.content-container.white-bg.animated
   .container
+    %h2.page-headline.text-center
+      Review Apps
+    %h3.getting-started-subheader.text-center
+      :markdown
+        Review Apps is available in GitLab.com, GitLab Community Edition, and GitLab Enterprise Edition.
     .row.row-eq-height
-      .col-md-6
-        = image_tag "/images/review-apps/review-apps-graph.png", class: "img-responsive"
-      .col-md-6
+      .col-md-6.feature-item-graphic
+        = image_tag "/images/review-apps/review-apps-graph.svg", class: "img-responsive"
+      .col-md-6.feature-item.step-content
         %p
           :markdown
             Review Apps transform the way you and your team work. For developers this