Improve the Multivariate UI in VEC to make it easier to create multivariates by code

[#1]

Currently it is extremely difficult to create a multivariate experiment in Target using code (either JavaScript or CSS changes). Currently you have to select a element and apply a HTML offer(s) to it.

Ideally there would be an easier to add code multivariate in the UI without the need to select an individual element to apply it to.