We were using the Draft4Validator in one place, so this updates it to
the Draft7Validator instead.
The schemas were mostly using draft 4 of the JSON schema, though there
were a couple of schemas that were already of version 7. This updates
them all to version 7, which is the latest version fully supported by
the jsonschema Python package. There are some breaking changes in the
newer version of the schema, but I could not see anywhere would these
affect us. Some of these schemas were not valid in version 4, but are
now valid in version 7 because `"required": []` was not valid in earlier
versions.
When a precompiled letter is sent via the admin app, we now pass in the address which can be set in the Notifications.to field.
Once a precompiled letters sent by the API has passed validation we can set the address in Notifications.to field.
The celery tasks to validate precompiled letters sent by the API will be done in another PR.