Onboarding - Profile Wizard: Implement Muriel styling for form components

[justinshreve]

The profile wizard uses Muriel components for form inputs, buttons, etc. These components exist in Gutenberg, but we need to implement our own layer of styling over these.

See p1556119186006300-slack-wc-onboarding for Figma links containing more information on the styles.

We will need styles for:

• Primary Button
• Secondary Button
• Checkboxes
• Form Inputs
• Dropdowns

In terms of implementation, we should be overriding these styles at the component level. See p1557235811036900-slack-nux. Newspack is also doing something similar. Example: Automattic/newspack-plugin#46.

On hold pending: #2166 (comment)

[justinshreve]

As a follow-up from Slack, we plan on importing components from the Newspack team to avoid code duplication. They have already written the exact form component set we need. See p1558027644118700-slack-newspack-a8c.

In the meantime, we can just use @wordpress/components directly, because the signature/props will remain the same.