AHVLA now APHA: changes needed to AHVLA and Defra

Users can no longer access AHVLA mappings, as they are now part of 
the new APHA organisation.  PR changes site ownership to new 
organisation and makes Defra mappings available to that new 
organisation.
Guess not.  

So just stick: 
```
extra_organisation_slugs:
- animal-health-and-veterinary-laboratories-agency
```
in defra_vla.yml?