OpenAPI schema endpoint

Requires !759 (merged) — closes #264 (closed)

This adds a new endpoint at /api/v1/schema/ that serves the OpenAPI schema. The custom SchemaGenerator has been updated to ensure that, unlike DRF's default behavior, endpoints that cannot be accessed by a user are still sent in schemas; this will allow the API client to access the schema immediately without requiring the user to login, or avoid re-downloading and patching its schema on login or logout. This also avoids issues with computed defaults that returned objects such as the local ImageServer as PyYAML !!python/object tags.