Add primary button to DIFM content form

[fditrapani]

Problem

After you purchase a DIFM package, you need to enter your content. Each step of the content flow has two buttons that are equally weighted:

Since we want people to continue, let's make the Next button primary, drop the arrow (we don't do that anywhere else), and tighten the gap between the buttons to 16px:

Where

Buy a DIFM package by selecting Build my site for me from the goals step when you create your site.

[github-actions]

OpenAI suggested the following labels for this issue:

• [Feature Group] DIFM (Do It For Me) Flows: The issue pertains to enhancing the user experience in the DIFM content form process.
• [Feature] Customer Seller Experience: The improvements aim to streamline the purchasing process and user engagement during content submission.
• [Feature] Design: The request involves design changes to button prominence which directly impacts the aesthetic and usability of the content form.