Style 'help' text for checkboxes

[claudiulodro]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Related #64

This PR adds styling for the 'help' text on checkboxes.

Here is the design for this:

Without the CSS in this PR, here is what it would look like:

How to test the changes in this Pull Request:

1. npm run clean; npm run build:webpack
2. Go to Components Demo. Examine the new 'Checkbox w/Help' element I've added:

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[jeffersonrabb]

LGTM