Pre-populate rename folder form with current name

Often people will be editing the existing name, not changing it
completely.

This matches what we do when someone wants to rename their template or
service.

app/main/views/templates.py

@@ -374,7 +374,10 @@ def manage_template_folder(service_id, template_folder_id):
     if not current_service.has_permission('edit_folders'):
         abort(403)
 
-    form = TemplateFolderForm()
+    form = TemplateFolderForm(
+        name=current_service.get_template_folder(template_folder_id)['name']
+    )
+
     if form.validate_on_submit():
         template_folder_api_client.update_template_folder(
             current_service.id, template_folder_id, name=form.name.data

app/models/service.py

@@ -1,3 +1,4 @@
+from flask import abort
 from notifications_utils.field import Field
 from werkzeug.utils import cached_property
 
@@ -307,6 +308,15 @@ class Service():
             )
         ]
 
+    def get_template_folder(self, folder_id):
+        try:
+            return next(
+                folder for folder in self.all_template_folders
+                if folder['id'] == folder_id
+            )
+        except StopIteration:
+            abort(404)
+
     def is_folder_visible(self, template_folder_id, template_type='all'):
 
         if template_type == 'all':

tests/app/main/views/test_template_folders.py

@@ -324,7 +324,17 @@ def test_get_manage_folder_page(client_request, service_one, mock_get_template_f
         service_id=service_one['id'],
         template_folder_id=folder_id
     )
-    assert page.select_one('input[name=name]') is not None
+    assert page.select_one('input[name=name]')['value'] == 'folder_two'
+
+
+def test_manage_folder_page_404s(client_request, service_one, mock_get_template_folders):
+    service_one['permissions'] += ['edit_folders']
+    client_request.get(
+        'main.manage_template_folder',
+        service_id=service_one['id'],
+        template_folder_id=str(uuid.uuid4()),
+        _expected_status=404,
+    )
 
 
 def test_get_manage_folder_page_no_permissions(client_request, service_one, mock_get_template_folders):