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 %}