task(settings): Support additional layout customization

[vpomerleau]

Because

• We want to allow setting a gradient as button colour
• We want to allow customization of headline text size and color

This pull request

• Update the CmsButtonWithFallback component to support setting a gradient as buttonColor
• Update calculate-contrast to provide better contrast on light colour gradient
• Update cta styling to prevent a border artifact when background is gradient
• Support headlineFontSize and headlineTextColor attributes from cmsInfo

Issue that this pull request solves

Closes: FXA-12791

Checklist

Put an x in the boxes that apply

• My commit is GPG signed.
• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).

Screenshots (Optional)

Example medium headline font size with blue text and gradient button

Button styling examples from storybook with automated text color adjustments based on contrast with button colour.

Other information (Optional)

Testing requires:

• task(content): Add new content types for account headline styling fxa-strapi#159
• fxa-strapi dev running locally
• relying party customization set up in strapi dev for local 123Done (dcdb5ae7add825d2) with entrypoint (e.g., fxa_avatar_menu)