We did not have a JSON schema for updating a template. Since we will
remove the postage constraint from the templates table, this adds a JSON
schema for updating a template so that we can use it to check that the
postage is one of the allowed values.
When creating the Tempalte from_json, the folder is passed in. Since some validation should done, as in the folder exists and is for the same service, the folder is passed through to the Tempalte.from_json method.
When the template is persisted so is the relationship to folders.
TODO: If the folder is invalid a specific message should be returned.
Updated jsonschema to Draft7, this allowed a conditional validation on subject, if template_type == 'email' or 'letter' then subject is required.
This version is backward compatible with Draft4.
When creating TempalteRedacted, I've built the dict depending on if the created_by or created_by_id exists.
