Add ability to license multiple groups

Problem/Motivation

Until now this module can only license one group per product variation.
The field where you select the group to license is an options radio that gives you only one option to select.

Steps to reproduce

Use the module as is without the patch in this issue.

Proposed resolution

I add a patch that adds ability to license multiple groups on the same product variation.

User interface changes

You get a select multiple field on product variation add/edit form.