Make a macro for email branding previews

This means we:
- don’t have to copy and paste the `<iframe>` code to every place where
  we use it
- can make changes to it in one place and have it apply everywhere

app/templates/components/branding-preview.html

@@ -0,0 +1,3 @@
+{% macro branding_preview(branding_style) %}
+  <iframe src="{{ url_for('main.email_template', branding_style=branding_style) }}" class="branding-preview"></iframe>
+{% endmacro %}

app/templates/views/organisations/organisation/settings/preview-email-branding.html

@@ -1,6 +1,7 @@
 {% extends "org_template.html" %}
 {% from "components/form.html" import form_wrapper %}
 {% from "components/button/macro.njk" import govukButton %}
+{% from "components/branding-preview.html" import branding_preview %}
 
 {% block org_page_title %}
   Preview email branding
@@ -11,7 +12,7 @@
   <h1 class="heading-large">Preview email branding</h1>
   <div class="govuk-grid-row">
     <div class="govuk-grid-column-full">
-      <iframe src="{{ url_for('main.email_template', branding_style=form.branding_style.data) }}" class="branding-preview"></iframe>
+      {{ branding_preview(form.branding_style.data) }}
       {% call form_wrapper(action=action) %}
         <div class="form-group">
           {{ form.hidden_tag() }}

app/templates/views/service-settings/branding/email-branding-govuk.html

@@ -3,6 +3,7 @@
 {% from "components/back-link/macro.njk" import govukBackLink %}
 {% from "components/page-footer.html" import page_footer %}
 {% from "components/page-header.html" import page_header %}
+{% from "components/branding-preview.html" import branding_preview %}
 
 {% block service_page_title %}
   Check your new branding
@@ -22,7 +23,7 @@
     Emails from {{ current_service.name }} will look like this.
   </p>
 
-  <iframe src="{{ url_for('main.email_template', branding_style='__NONE__') }}" class="branding-preview"></iframe>
+  {{ branding_preview('__NONE__') }}
 
   <h2 class="heading-medium">Before you continue</h2>
 

app/templates/views/service-settings/branding/email-branding-nhs.html

@@ -3,6 +3,7 @@
 {% from "components/back-link/macro.njk" import govukBackLink %}
 {% from "components/page-footer.html" import page_footer %}
 {% from "components/page-header.html" import page_header %}
+{% from "components/branding-preview.html" import branding_preview %}
 
 {% block service_page_title %}
   Check your new branding
@@ -21,8 +22,7 @@
   <p class="govuk-body">
     Emails from {{ current_service.name }} will look like this.
   </p>
-
-  <iframe src="{{ url_for('main.email_template', branding_style=nhs_branding_id) }}" class="branding-preview"></iframe>
+  {{ branding_preview(nhs_branding_id) }}
 
   <h2 class="heading-medium">Before you continue</h2>
 

app/templates/views/service-settings/branding/email-branding-options.html

@@ -6,6 +6,7 @@
 {% from "components/page-footer.html" import page_footer %}
 {% from "components/form.html" import form_wrapper %}
 {% from "components/back-link/macro.njk" import govukBackLink %}
+{% from "components/branding-preview.html" import branding_preview %}
 
 {% block service_page_title %}
   Change email branding
@@ -25,8 +26,9 @@
     Your emails currently have {{ branding_name }} branding.
   </p>
 
-  {% set branding_id = current_service.email_branding_id if current_service.email_branding else '__NONE__' %}
-  <iframe src="{{ url_for('main.email_template', branding_style=branding_id) }}" class="branding-preview"></iframe>
+  {{ branding_preview(
+    current_service.email_branding_id if current_service.email_branding else '__NONE__'
+  ) }}
 
   {% if current_service.needs_to_change_email_branding %}
     <p class="govuk-body">

app/templates/views/service-settings/preview-email-branding.html

@@ -1,6 +1,7 @@
 {% extends "withnav_template.html" %}
 {% from "components/form.html" import form_wrapper %}
 {% from "components/button/macro.njk" import govukButton %}
+{% from "components/branding-preview.html" import branding_preview %}
 
 {% block service_page_title %}
   Preview email branding
@@ -11,7 +12,7 @@
   <h1 class="heading-large">Preview email branding</h1>
   <div class="govuk-grid-row">
     <div class="govuk-grid-column-full">
-      <iframe src="{{ url_for('main.email_template', branding_style=form.branding_style.data) }}" class="branding-preview"></iframe>
+      {{ branding_preview(form.branding_style.data) }}
       {% call form_wrapper(action=action) %}
         <div class="form-group">
           {{ form.hidden_tag() }}