add template folder CRUD

* create template folder
* rename template folder
* get list of template folders for service (not nested/presented in any
  particular way)
* delete template folder

Also removed `lazy=dynamic` from the `template_folder.templates`
relationship. lazy=dynamic returns a query object (which you can then
filter further). We just want to return the entire fetched list, at
least for now.

app/schema_validation/definitions.py

@@ -11,6 +11,14 @@ uuid = {
     "link": "link to our error documentation not yet implemented"
 }
 
+nullable_uuid = {
+    "type": ["string", "null"],
+    "format": "validate_uuid",
+    "validationMessage": "is not a valid UUID",
+    "code": "1001",  # yet to be implemented
+    "link": "link to our error documentation not yet implemented"
+}
+
 
 personalisation = {
     "type": "object",