Convert BaseTemplateForm to use new fields

Updates StringField to GovukTextInputField.

These changes also affect the following forms that
inherit from BaseTemplateForm:
- SMSTemplateForm
- EmailTemplateForm

Includes changes to templates that use these forms
and associated tests.

app/templates/views/edit-email-template.html

@@ -19,7 +19,10 @@
     {% call form_wrapper() %}
       <div class="govuk-grid-row">
         <div class="govuk-grid-column-five-sixths">
-          {{ textbox(form.name, width='1-1', hint='Your recipients will not see this', rows=10) }}
+          {{ form.name(param_extensions={
+            "classes": "govuk-!-width-full",
+            "hint": {"text": "Your recipients will not see this"}
+          }) }}
           {{ textbox(form.subject, width='1-1', highlight_placeholders=True, rows=2) }}
           {{ textbox(form.template_content, highlight_placeholders=True, width='1-1', rows=8) }}
           {% if current_user.platform_admin %}