Improve merge request version switcher

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/assets/stylesheets/framework/dropdowns.scss

@@ -17,6 +17,12 @@
 
 .dropdown {
   position: relative;
+
+  .btn-link {
+    &:hover {
+      cursor: pointer;
+    }
+  }
 }
 
 .open {

app/assets/stylesheets/pages/merge_requests.scss

@@ -371,3 +371,9 @@
     }
   }
 }
+
+.mr-version-switch {
+  background: $background-color;
+  padding: $gl-btn-padding;
+  border-bottom: 1px solid $border-color;
+}

app/views/projects/merge_requests/show/_versions.html.haml

-- if @merge_request.merge_request_diffs.size > 1
-  - latest_diff = @merge_request.merge_request_diff
-  .light-well
-    Merge Request version
+- diffs_count = @merge_request.merge_request_diffs.count
 
+- if diffs_count > 1
+  - latest_diff = @merge_request.merge_request_diff
+  .mr-version-switch
+    Version: 
     %span.dropdown.inline
-      %a.dropdown-toggle{ data: {toggle: :dropdown} }
-        %strong.monospace #{@merge_request_diff.base_commit.short_id}..#{@merge_request_diff.head_commit.short_id}
+      %a.btn-link.dropdown-toggle{ data: {toggle: :dropdown} }
+        %strong.monospace<
+          - if latest_diff == @merge_request_diff
+            #{"latest"}
+          - else
+            #{@merge_request_diff.base_commit.short_id}..#{@merge_request_diff.head_commit.short_id}
         %span.caret
-      %ul.dropdown-menu
+      %ul.dropdown-menu.dropdown-menu-selectable
         - @merge_request.merge_request_diffs.each do |merge_request_diff|
-          %li{ class: ('active' if merge_request_diff == latest_diff) }
-            = link_to diffs_namespace_project_merge_request_path(@project.namespace, @project, @merge_request, diff_id: merge_request_diff.id) do
+          %li
+            = link_to diffs_namespace_project_merge_request_path(@project.namespace, @project, @merge_request, diff_id: merge_request_diff.id), class: ('is-active' if merge_request_diff == @merge_request_diff) do
               %strong.monospace
                 #{merge_request_diff.base_commit.short_id}..#{merge_request_diff.head_commit.short_id}
               %br
               %small
+
                 = time_ago_with_tooltip(merge_request_diff.created_at)