Return rebase-before-merge checkbox

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

app/views/projects/merge_requests/widget/open/_accept.html.haml

@@ -9,6 +9,12 @@
         = label_tag :should_remove_source_branch, class: "remove_source_checkbox" do
           = check_box_tag :should_remove_source_branch
           Remove source-branch
+    - if @merge_request.target_project.merge_requests_rebase_enabled && can_rebase?(@merge_request.target_project, @merge_request.target_branch)
+      .accept-control.remove_branch_holder.checkbox
+        = label_tag :should_rebase do
+          = check_box_tag :should_rebase, "1", @project.merge_requests_rebase_default
+          %strong Rebase before merge
+
     .accept-control
       = link_to "#", class: "modify-merge-commit-link js-toggle-button", title: "Modify merge commit message" do
         %i.fa.fa-edit