Billboard option for 'Y-only' doesn't make sense in a Blender context

[j-conrad]

Description
Because it's somewhat expected to use an Empty (usually the 'Arrows' type) for things like the Billboard component, I'm finding it confusing that the component has an option called Only Y since Blender uses Z-up and the Empty itself clearly displays Z as the up axis.
I know it's Only Y because that's the final (threejs) axis used in Hubs. But users don't know that--especially non-Spoke folks.

Proposed Change
The option might be better understood if it read Vertical Axis Only

I understand that someone could change the orientation of the object and become confused about which axis is the 'vertical' one, but for the purposes of creating sensible defaults and template objects, this proposed change will likely reduce overall confusion.

[hrithikwins]

@keianhzo @Exairnous I'm interested to contribute on this issue