Import as its Own Endpoint

There may be some small advantage to offering an `/import` endpoint when someone explicitly intends to use it instead of having to `/update`. This would require it's own RESTful checks and responses.

An `/import` is a perfect mix of `/create` and `/update`. It "updates an item in the graph" by "creating a new root" in RERUM which references the origin URI through __rerum.history.previous.

I think this endpoint would be a POST request which responds 201 on success. Currently, it is behind PUT `/update` and responds 200 upon success. Something to think about if we decide to do this.