Split schema files and register signup artefact with panopticon

Finders have both metadata (used for registering the artefact with panopticon) and a schema (used to build the finder).

Previously, all this information was held in the one file -- the schema. Now, it's held in two separate files -- `metadata/:slug.json` for the data that should go on the artefact, and `schema/:slug.json` for the
schema itself.

This makes it easier to add a 'signup' artefact which uses information which shouldn't be present in the schema.