move folders and templates to other folders

new endpoints:
/services/<service_id>/move-to-folder
/services/<service_id>/move-to-folder/<target_template_folder_id>

* takes in a dict containing lists of `templates` and `folders` uuids.
* sets parent of templates and folders to the folder specified in the
  URL. Or None, if there was no id specified.
* if any template or folder has a differen service id, then the whole
  update fails
* if any folder is an ancestor of the target folder, then the whole
  update fails (as that would cause a cyclical folder structure).
* the whole function is wrapped in a single `transactional` decorator,
  so in case of error nothing will be saved.

app/models.py

@@ -725,6 +725,13 @@ class TemplateFolder(db.Model):
             'service_id': self.service_id
         }
 
+    def is_parent_of(self, other):
+        while other.parent is not None:
+            if other.parent == self:
+                return True
+            other = other.parent
+        return False
+
 
 template_folder_map = db.Table(
     'template_folder_map',