Add a test to verify that translated labels work

Problem:

There was uncertainty about how to create custom labels for fields.
In #451,
it was suggested that we could use I18n translations
to solve the problem.

If we accept that as the standard way to set custom labels,
we should have a test for it.

Solution:

- Add a test to verify that labels can be customized through I18n.
- Add documentation about how to customize labels

To Do:

- Update view code so the same translation definition
  applies on the show and collection pages,
  in addition to the form page.

docs/customizing_dashboards.md

@@ -76,4 +76,15 @@ if the value is stored by the number of cents:
   )
 ```
 
+To change the user-facing label for an attribute,
+define a custom I18n translation:
+
+```yaml
+en:
+  helpers:
+    label:
+      customer:
+        name: Full Name
+```
+
 [define your own]: /adding_custom_field_types

spec/features/form_spec.rb

@@ -8,4 +8,24 @@
 
     expect(page).to have_css("form.form")
   end
+
+  it "displays translated labels" do
+    custom_label = "Newsletter Subscriber"
+
+    translations = {
+      helpers: {
+        label: {
+          customer: {
+            email_subscriber: custom_label,
+          },
+        },
+      },
+    }
+
+    with_translations(:en, translations) do
+      visit new_admin_customer_path
+
+      expect(page).to have_label(custom_label)
+    end
+  end
 end

spec/support/features/page_elements.rb

@@ -2,4 +2,8 @@ module Features
   def have_header(title)
     have_css("h1", text: title)
   end
+
+  def have_label(title)
+    have_css("label", text: title)
+  end
 end