Update AddGPOrganisationForm with new fields

Changes its StingFields to GovukTextInputFields.

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

app/main/forms.py

@@ -1061,7 +1061,7 @@ class AddGPOrganisationForm(StripWhitespaceForm):
         ),
     )
 
-    name = StringField(
+    name = GovukTextInputField(
         'What’s your practice called?',
     )
 

app/templates/views/organisations/add-gp-organisation.html

@@ -22,7 +22,7 @@
       {% endfor %}
     {% endcall %}
     {% call conditional_radio_panel('custom-organisation-name') %}
-      {{ textbox(form.name) }}
+      {{ form.name }}
     {% endcall %}
     {{ page_footer('Continue') }}
   {% endcall %}