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.
To test go to https://www.optimizely.de/pricing, and in the console copy and past all of the jQuery extension code below.  Also, apply 

```css
.plan-box-cont {
  display: flex;
  flex-direction: column;
}
.plan-box-content {
  flex: 1;
}
```

Then in the console call `$('.plan-box .cta').equalHeightGrid;`