Warn users a template change will break things

When a user adds or removes placeholders in their template we should consider
this a ‘breaking change’ and warn them accordingly.

Implementing this mostly relies on using
https://github.com/alphagov/notifications-utils/pull/37

Temporarily storing the new template until the user confirms that they want to
make the changes in done using hidden fields. This is a bit hacky, but the
complexity of making sessions interact with WTForms was just too much to handle.

This commit also changes the example spreadsheet that we show on this page to
look more like a spreadsheet.

app/main/views/templates.py

@@ -1,12 +1,16 @@
-from flask import request, render_template, redirect, url_for, flash, abort
+import string
+
+from flask import request, render_template, redirect, url_for, flash, abort, session
 from flask_login import login_required
 
-from notifications_utils.template import Template
+from notifications_utils.template import Template, TemplateChange
+from notifications_utils.recipients import first_column_heading
 from notifications_python_client.errors import HTTPError
 
 from app.main import main
 from app.utils import user_has_permissions
 from app.main.forms import SMSTemplateForm, EmailTemplateForm
+from app.main.views.send import get_example_csv_rows
 from app import service_api_client, current_service
 
 
@@ -109,6 +113,28 @@ def edit_service_template(service_id, template_id):
     form = form_objects[template['template_type']](**template)
 
     if form.validate_on_submit():
+        subject = form.subject.data if getattr(form, 'subject', None) else None
+        new_template = Template({
+            'name': form.name.data,
+            'content': form.template_content.data,
+            'subject': subject,
+            'template_type': template['template_type'],
+            'id': template['id']
+        })
+        template_change = Template(template).compare_to(new_template)
+        if template_change.has_different_placeholders and not request.form.get('confirm'):
+            return render_template(
+                'views/templates/breaking-change.html',
+                template_change=template_change,
+                new_template=new_template,
+                column_headings=list(string.ascii_uppercase[:len(new_template.placeholders) + 1]),
+                example_rows=[
+                    [first_column_heading[new_template.template_type]] + list(new_template.placeholders),
+                    get_example_csv_rows(new_template),
+                    get_example_csv_rows(new_template)
+                ],
+                form=form
+            )
         try:
             service_api_client.update_service_template(
                 template_id,
@@ -116,7 +142,7 @@ def edit_service_template(service_id, template_id):
                 template['template_type'],
                 form.template_content.data,
                 service_id,
-                form.subject.data if getattr(form, 'subject', None) else None
+                subject
             )
         except HTTPError as e:
             if e.status_code == 400: