Merge branch 'scroll-to-changes-content' into 'master'

Clicking changes tab scrolls the window to show the content

When you click on the changes tab in a merge request, it will now scroll the window down to the content.

![scroll](/uploads/c5db23e1c32b1fe62bf4efb815b3f055/scroll.gif)

Closes #15133

See merge request !4192

app/assets/javascripts/merge_request_tabs.js.coffee

@@ -75,6 +75,9 @@ class @MergeRequestTabs
       @loadDiff($target.attr('href'))
       if bp? and bp.getBreakpointSize() isnt 'lg'
         @shrinkView()
+
+      navBarHeight = $('.navbar-gitlab').outerHeight()
+      $.scrollTo(".merge-request-details .merge-request-tabs", offset: -navBarHeight)
     else if action == 'builds'
       @loadBuilds($target.attr('href'))
       @expandView()