CheckoutPane::buildPaneSummary() should return renderable arrays

We should add some code for BC so that if buildPaneSummary of a pane returns a string, we wrap it into a '#type' => 'markup' element. That way we don't break existing panes.

-    return $this->order->getEmail();
+    return [
+      '#type' => 'inline_template',
+      '#template' => '{{ email }}',
+      '#context' => [
+        'email' => $this->order->getEmail(),
+      ],
+    ];

What is the reasoning for using an inline_template here? I'd expect a markup element.

Here's an updated patch.

PR https://github.com/drupalcommerce/commerce/pull/644

Thanks, guys.