Hide backend new card form by default

When the page first loads, this form should not be visible. We want it
to be displayed when the user selects "new card". Otherwise it's visible
when a reusable source is selected.

app/views/spree/admin/payments/source_forms/_paypal_braintree.html.erb

@@ -10,5 +10,7 @@
     <% end %>
   </div>
 
-  <%= render partial: "spree/shared/braintree_hosted_fields", locals: { id: id } %>
+  <div id="card_form<%= id %>" data-hook style="display: none;">
+    <%= render partial: "spree/shared/braintree_hosted_fields", locals: { id: id } %>
+  </div>
 </fieldset>

app/views/spree/shared/_braintree_hosted_fields.html.erb

@@ -1,6 +1,6 @@
 <% prefix = "payment_source[#{id}]" %>
 
-<div id="card_form<%= id %>" data-hook>
+<div class="hosted-fields">
   <div class="field" data-hook="card_number">
     <%= label_tag "card_number#{id}", Spree::CreditCard.human_attribute_name(:number), class: "required" %>
     <div class="input" id="card_number<%= id %>"></div>

spec/features/braintree_credit_card_checkout_spec.rb

@@ -22,7 +22,7 @@
     visit spree.checkout_state_path(:delivery)
     click_button "Save and Continue"
     choose("Braintree")
-    expect(page).to have_selector("#card_form#{braintree.id}", visible: true)
+    expect(page).to have_selector("#payment_method_#{braintree.id}", visible: true)
     expect(page).to have_selector("iframe#braintree-hosted-field-number")
   end
 end