Add emmet-mode to react-mode

[kaipr]

This enables emmet-mode in react-mode (*.jsx files).
It automatically uses className instead of class, so no other configuration needed.

Put it there because thats how other modes are hooked, could also add it as a hook in the react layer if that would be preferred?

Closes #5130

[TheBB]

Thanks! Cherry-picked in develop. You can safely delete your branch.

I moved it to the react layer.

[NdYAG]

@kaipr @TheBB
Should we also delete this line?
https://github.com/syl20bnr/spacemacs/blob/master/layers/+frameworks/react/packages.el#L87