Extend thoth-loader to handle SciELO JSON import

[brendan-oconnell]

SciELO metadata was previously added to Thoth using ONIX, but future ingests will use JSON, which contains additional metadata not contained in ONIX.

• [x] Create new model based on existing ONIX import module
• [x] Add additional data from JSON not included in ONIX
• [x] Add logic to import any of this data which is supported by Thoth
• [x] Complete Book Loader
• [x] Test import locally, check over created records, fix/iterate if required
• [x] Submit draft PR for Book Loader
• [x] Make changes based on PR and resubmit
• [x] Complete Chapter Loader
• [x] Submit draft PR for Chapter Loader
• [x] Make changes based on PR and resubmit
• [ ] Run live import (including preparation e.g. backup of existing data if required)