Split schema into separate schema and metadata files

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.
Register Signup artefact with panopticon

For each finder, using its metadata hash, register a signup artefact
with panopticon.
Split presenters into separate files
Modify PanopticonRegisterer to be instantiatable
Merge pull request #55 from alphagov/split-schema-files

Register email signup artefact for each finder with Panopticon