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/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':