Post-Signup Interstitial: Create interstitial screen

[renanferrari]

Designs

Standard	Dark Mode
Landscape
Tablet

Tasks

• Create layout
  • Standard
  • Dark Mode
  • Landscape
  • Tablet
• Implement basic functionality
  • Make Create New Site button open its respective screen
    • Ensure the site creation flow completes successfully (moved to next task)
  • Make Add a Self-Hosted Site button open its respective screen
    • Ensure the site connection flow completes successfully (moved to next task)
  • Make Not Right Now button open the main screen
    • Ensure it redirects to the Reader tab

Update: Added new design spec for landscape layout

Notes

• Master Issue: Post-Signup Interstitial Master Issue #10918

[renanferrari]

• The dark mode layout has been scraped for now, as it would depend on the work being done at the material-theme branch.
• There's a new design spec for landscape mode on phones, which I'll be adding here soon.