Convert IntegerFields to GovukIntegerFields

Changes those fields in the following forms:
- FreeSMSAllowance
- ProviderForm
- ProviderRatioForm
- ServiceDataRetentionEditForm

Includes changes to templates that use this form
and associated tests.

app/templates/views/providers/edit-provider.html

@@ -25,7 +25,7 @@ Provider - {{provider.display_name}}
         </ul>
 
         {% call form_wrapper() %}
-            {{ textbox(form.priority) }}
+            {{ form.priority }}
             {{ page_footer('Save') }}
         {% endcall %}
 

app/templates/views/service-settings/data-retention/add.html

@@ -18,7 +18,7 @@
 
     {% call form_wrapper() %}
       {{ radios(form.notification_type) }}
-      {{ textbox(form.days_of_retention) }}
+      {{ form.days_of_retention }}
       {{ page_footer('Add') }}
     {% endcall %}
 

app/templates/views/service-settings/data-retention/edit.html

@@ -17,7 +17,7 @@
     ) }}
     {% call form_wrapper() %}
       {{ notification_type | capitalize}}
-      {{  textbox(form.days_of_retention) }}
+      {{  form.days_of_retention }}
       {{ page_footer('Save') }}
     {% endcall %}
 

app/templates/views/service-settings/set-free-sms-allowance.html

@@ -15,7 +15,7 @@
       'Free text message allowance',
       back_link=url_for('.service_settings', service_id=current_service.id)
     ) }}
-    {{ textbox(form.free_sms_allowance) }}
+    {{ form.free_sms_allowance }}
     {{ page_footer('Save') }}
   {% endcall %}