fix: Fix spacing between form elements for "small" variant

A fix for the issue described in this ticket - #36 - that describes how spacing between headline and copy elements, and, alternately, between headline and page elements, is determined for "small" form elements (as the ticket puts it).

In particular, this effects type lockups where form labels sit atop form fields, and where label-style type sits atop tiny-style copy (such as in the captcha).

This change was implemented in SASS and HTML by:

• Adding a SASS partial for the CAPTCHA and a class to the CAPTCHA headline to allow for more direct control of styling
• Adjusting the grid spacing between columns containing standard form fields
• Adjusting the margin above the newsletter signup checkbox and label

Closes: #36