Adds a new management command to load OTM1 species

Since OTM2 and OTM1 species are largely compatible, it makes sense to
allow devs to load an existing species list. The command is idempotent,
running it multiple times wont make a difference.

Example usage:

```bash
# On OTM1 instance
python manage.py dumpdata treemap.species > /tmp/species.json

# On OTM2 instance
python manage load_species /tmp/species.json
```