Split out source-layers and data-sources

This issue describes the re-organization of the static data used by the API.  

- [ ] Delete `data/layer-groups`
- [ ] Migrate discrete source-layers to `data/source-layers`
- [ ] Migrate source meta to `data/data-sources`
- [ ] Add Joi schemas for `source-layers` and `data-sources`
- [ ] Add tests to validate all `source-layers` and `data-sources`
- [ ] Delete `data/sources`