Use YAML Tags in blueprint export

Currently exported blueprints use primary keys verbatim, which makes it very difficult to re-use the export without manual editing. Ideally, the export should use !Find and !KeyOf to dynamically reference objects

For !Find, we should either look at what fields are unique in the database for a given model, and/or have a prioritised list of fields to try (`managed`, `slug`, `name`, `pk`)