Apply new GitLab UI for create mr button from issue

app/views/projects/issues/_new_branch.html.haml

@@ -57,7 +57,7 @@
               %span.js-ref-message.form-text.text-muted
             .form-group
-              %button.btn.btn-success.js-create-target{ type: 'button', data: { action: 'create-mr' } }
+              %button.btn.gl-button.btn-success.js-create-target{ type: 'button', data: { action: 'create-mr' } }
                 = create_mr_text
             - if can_create_confidential_merge_request?