Add-on needs to respect online access permissions

[BradyAJohnston]

Any online access should be limited behind bpy.app.online_access, which is a preference that allows for online access.

https://docs.blender.org/manual/en/dev/advanced/extensions/addons.html#internet-access

The only online access is currently downloading from the PDB. Any operators that use this should be disabled if online access disabled, with informative errors / warnings around this also.