Equal height utils function for DE pricing grid vertical alignment

This PR addresses MKT-828, https://github.com/optimizely/marketing-website/pull/726, https://github.com/optimizely/marketing-website/pull/738.

Testing instructions are below.  Note that the responsive version of this function does not work for the Pricing page because our grid elements we are targeting are in columns and not rows.
I get equal height content boxes when I do 
```
window.optly.mrkt.utils.equalHeightGrid({
  selector: '.plan-box-content'
});
```
Could you confirm? What is currently there lengthens both boxes.