Studio: Handle Add Site top bar menu item in onboarding screen

[katinthehatsite]

Closes https://github.com/Automattic/studio/issues/377

Proposed Changes

This PR changes the behaviour of the Add site menu item in the top bar menu when the user is presented with the onboarding screen. Instead of doing nothing, the menu item now triggers the site creation.

Testing Instructions

• Pull the changes from this branch
• Start the app with npm start
• Delete all local sites to be presented with the onboarding screen
• Once on onboarding screen, click on Add Site.. in the top bar menu
• Observe that it creates a new site and you are presented with the loading screen
• Observe that once the site is created and you try to click on Add Site..., it opens a modal with add site form

Pre-merge Checklist

• [ ] Have you checked for TypeScript, React or other console errors?

[kozer]

@katinthehatsite , I left a comment. Other than that, it works as expected! Nice work!