Create project artefacts

Implement a table/model to allow projects to have artefacts. Artefacts are any sort of document (PDF, image, text, zip, etc) that can be attached to the project.

This task covers the basic implementation of this table/model.

The issue names no files, tests, or entry points. Start by examining existing project models and database-table conventions; done means a basic artefact table/model exists and supports documents attached to projects, including PDFs, images, text, and ZIP files.

Written by the indexing model from the issue text.