Updated grid classes to include 'xl' for largest viewport.

[ray760]

What's this PR do?

This pull request adds "xl" gird class for largest viewports

Where should the reviewer start?

Download branch and check boilerplate index.html file. Check added cbp-grid-col-6-xl class, and also play around with different column widths to see if there are any issues.

How should this be manually tested?

Yes

Any background context you want to provide?

N/A

What are the relevant tickets?

N/A

Screenshots (if appropriate)

N/A

Questions:

• Is there a blog post? N/A
• Does the knowledge base need an update? N/A