feature: Landing page styling

[ShrimpCryptid]

Implements styling of the landing page, Pt. II of #188. I expect to maybe have 1-2 more PRs before this issue is fully closed.

Estimated size: medium, 20 minutes.

Changes:

• Adds the banner video MP4, which autoplays when loaded.
  • Also added an accessibility feature, where the video playback is disabled if prefers-reduced-motion is set to true.
• Adds Open Sans via Google fonts. Existing fonts are still included to ensure compatibility with embedded use cases, but will be removed in the future.
• Adds the StyleProvider component, which will wrap the LandingPage and App in the future to provide a central location for CSS variables.
• Implements styling for the landing page.

Design	Implementation

Thanks for contributing!