Show letter branding preview when selecting a brand

This adds a preview pane which is visible when updating a letter brand.

If JavaScript is enabled, the preview pane shows on the set-letter-branding
page, and submitting the form saves updates the letter brand for a service
immediately. If Javascript is not enabled, there is a separate 'Preview email
branding' page which shows a preview of the brand and has a 'Save' button on it.

app/main/views/index.py

@@ -10,9 +10,9 @@ from flask_login import current_user, login_required
 from notifications_utils.international_billing_rates import (
     INTERNATIONAL_BILLING_RATES,
 )
-from notifications_utils.template import HTMLEmailTemplate
+from notifications_utils.template import HTMLEmailTemplate, LetterImageTemplate
 
-from app import email_branding_client
+from app import email_branding_client, letter_branding_client
 from app.main import main
 from app.main.forms import FieldWithNoneOption, SearchByNameForm
 from app.main.views.sub_navigation_dictionaries import features_nav
@@ -174,6 +174,35 @@ def email_template():
     return resp
 
 
+@main.route('/_letter')
+def letter_template():
+    branding_style = request.args.get('branding_style')
+
+    if branding_style == FieldWithNoneOption.NONE_OPTION_VALUE:
+        branding_style = None
+
+    if branding_style:
+        filename = letter_branding_client.get_letter_branding(branding_style)['filename']
+    else:
+        filename = 'no-branding'
+
+    template = {'subject': '', 'content': ''}
+    image_url = url_for('main.letter_branding_preview_image', filename=filename)
+
+    template_image = str(LetterImageTemplate(
+        template,
+        image_url=image_url,
+        page_count=1,
+    ))
+
+    resp = make_response(
+        render_template('views/service-settings/letter-preview.html', template=template_image)
+    )
+
+    resp.headers['X-Frame-Options'] = 'SAMEORIGIN'
+    return resp
+
+
 @main.route('/documentation')
 def documentation():
     return render_template('views/documentation.html')

app/main/views/service_settings.py

@@ -810,6 +810,28 @@ def service_set_letter_branding(service_id):
         current_branding=current_service.letter_branding_id,
     )
 
+    if form.validate_on_submit():
+        return redirect(url_for(
+            '.service_preview_letter_branding',
+            service_id=service_id,
+            branding_style=form.branding_style.data,
+        ))
+
+    return render_template(
+        'views/service-settings/set-letter-branding.html',
+        form=form,
+        search_form=SearchByNameForm()
+    )
+
+
+@main.route("/services/<service_id>/service-settings/preview-letter-branding", methods=['GET', 'POST'])
+@login_required
+@user_is_platform_admin
+def service_preview_letter_branding(service_id):
+    branding_style = request.args.get('branding_style')
+
+    form = ServicePreviewBranding(branding_style=branding_style)
+
     if form.validate_on_submit():
         current_service.update(
             letter_branding=form.branding_style.data
@@ -817,9 +839,10 @@ def service_set_letter_branding(service_id):
         return redirect(url_for('.service_settings', service_id=service_id))
 
     return render_template(
-        'views/service-settings/set-letter-branding.html',
+        'views/service-settings/preview-letter-branding.html',
         form=form,
-        search_form=SearchByNameForm()
+        service_id=service_id,
+        action=url_for('main.service_preview_letter_branding', service_id=service_id),
     )
 
 

app/main/views/templates.py

@@ -36,6 +36,7 @@ from app.utils import (
     get_template,
     should_skip_template_page,
     user_has_permissions,
+    user_is_platform_admin,
 )
 
 form_objects = {
@@ -222,6 +223,34 @@ def view_letter_template_preview(service_id, template_id, filetype):
     return TemplatePreview.from_database_object(db_template, filetype, page=request.args.get('page'))
 
 
+@main.route("/templates/letter-preview-image/<filename>")
+@login_required
+@user_is_platform_admin
+def letter_branding_preview_image(filename):
+    template = {
+        'subject': 'An example letter',
+        'content': (
+            'Lorem Ipsum is simply dummy text of the printing and typesetting '
+            'industry.\n\nLorem Ipsum has been the industry’s standard dummy '
+            'text ever since the 1500s, when an unknown printer took a galley '
+            'of type and scrambled it to make a type specimen book.\n\n'
+            '# History\n\nIt has survived not only\n\n'
+            '* five centuries\n'
+            '* but also the leap into electronic typesetting\n\n'
+            'It was popularised in the 1960s with the release of Letraset '
+            'sheets containing Lorem Ipsum passages, and more recently with '
+            'desktop publishing software like Aldus PageMaker including '
+            'versions of Lorem Ipsum.\n\n'
+            'The point of using Lorem Ipsum is that it has a more-or-less '
+            'normal distribution of letters, as opposed to using ‘Content '
+            'here, content here’, making it look like readable English.'
+        )
+    }
+    filename = None if filename == 'no-branding' else filename
+
+    return TemplatePreview.from_example_template(template, filename)
+
+
 def _view_template_version(service_id, template_id, version, letters_as_pdf=False):
     return dict(template=get_template(
         service_api_client.get_service_template(service_id, template_id, version=version)['data'],