Since template folders are only linked by ID to their parent we need
to check that the parent folder belongs to the same service as the
one being created. Otherwise, admin users could modify parent ID to
create a folder outside their service.
Ideally, this check would be performed by a DB constraint, but since
parent_id can be nullable this is only possible to express using DB
triggers.
Instead, we perform the check in the API endpoint code.
* 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.
