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.

tests/app/test_model.py

@@ -28,7 +28,8 @@ from tests.app.db import (
     create_inbound_number,
     create_reply_to_email,
     create_letter_contact,
-    create_template
+    create_template,
+    create_template_folder
 )
 
 
@@ -329,3 +330,17 @@ def test_is_precompiled_letter_hidden_true_not_name(sample_letter_template):
 def test_is_precompiled_letter_name_correct_not_hidden(sample_letter_template):
     sample_letter_template.name = PRECOMPILED_TEMPLATE_NAME
     assert not sample_letter_template.is_precompiled_letter
+
+
+def test_template_folder_is_parent(sample_service):
+    x = None
+    folders = []
+    for i in range(5):
+        x = create_template_folder(sample_service, name=str(i), parent=x)
+        folders.append(x)
+
+    assert folders[0].is_parent_of(folders[1])
+    assert folders[0].is_parent_of(folders[2])
+    assert folders[0].is_parent_of(folders[4])
+    assert folders[1].is_parent_of(folders[2])
+    assert not folders[1].is_parent_of(folders[0])