Use Template to replace/highlight placeholders

This commit brings in the `Template` util, added here:
https://github.com/alphagov/notifications-utils/pull/1

It also does a fair bit of tidying up, which I’ve unfortunately squashed into
this one massive commit. The main change is moving 404 handling into the
templates dao, so that every view isn’t littered with `try: … except(HTTPError)`.

It also adds new features, in a prototypy sort of way, which are:
- download a prefilled example CSV
- show all the columns for your template on the 'check' page

app/main/views/templates.py

@@ -1,13 +1,15 @@
 from flask import request, render_template, redirect, url_for, flash, abort
 from flask_login import login_required
 
+from notifications_python_client.errors import HTTPError
+from utils.template import Template
+
 from app.main import main
 from app.main.forms import TemplateForm
 from app import job_api_client
 from app.main.dao.services_dao import get_service_by_id
 from app.main.dao import templates_dao as tdao
 from app.main.dao import services_dao as sdao
-from notifications_python_client.errors import HTTPError
 
 
 @main.route("/services/<service_id>/templates")
@@ -15,7 +17,6 @@ from notifications_python_client.errors import HTTPError
 def manage_service_templates(service_id):
     try:
         jobs = job_api_client.get_job(service_id)['data']
-        templates = tdao.get_service_templates(service_id)['data']
     except HTTPError as e:
         if e.status_code == 404:
             abort(404)
@@ -25,7 +26,11 @@ def manage_service_templates(service_id):
         'views/manage-templates.html',
         service_id=service_id,
         has_jobs=bool(jobs),
-        templates=[tdao.TemplatesBrowsableItem(x) for x in templates])
+        templates=[
+            Template(template)
+            for template in tdao.get_service_templates(service_id)['data']
+        ]
+    )
 
 
 @main.route("/services/<service_id>/templates/add", methods=['GET', 'POST'])
@@ -56,14 +61,7 @@ def add_service_template(service_id):
 @main.route("/services/<service_id>/templates/<int:template_id>", methods=['GET', 'POST'])
 @login_required
 def edit_service_template(service_id, template_id):
-    try:
-        template = tdao.get_service_template(service_id, template_id)['data']
-    except HTTPError as e:
-        if e.status_code == 404:
-            abort(404)
-        else:
-            raise e
-
+    template = tdao.get_service_template_or_404(service_id, template_id)['data']
     template['template_content'] = template['content']
     form = TemplateForm(**template)
 
@@ -84,21 +82,14 @@ def edit_service_template(service_id, template_id):
 @main.route("/services/<service_id>/templates/<int:template_id>/delete", methods=['GET', 'POST'])
 @login_required
 def delete_service_template(service_id, template_id):
-    try:
-        template = tdao.get_service_template(service_id, template_id)['data']
-    except HTTPError as e:
-        if e.status_code == 404:
-            abort(404)
-        else:
-            raise e
-
-    template['template_content'] = template['content']
-    form = TemplateForm(**template)
+    template = tdao.get_service_template_or_404(service_id, template_id)['data']
 
     if request.method == 'POST':
         tdao.delete_service_template(service_id, template_id)
         return redirect(url_for('.manage_service_templates', service_id=service_id))
 
+    template['template_content'] = template['content']
+    form = TemplateForm(**template)
     flash('Are you sure you want to delete ‘{}’?'.format(form.name.data), 'delete')
     return render_template(
         'views/edit-template.html',