#6875599 Plugin Upload: new flow

Changes proposed in this Pull Request

The following changes depend on the marketplace feature flag which is true in all environments except production

After Users drop a file in the plugin upload dropzone http://calypso.localhost:3000/plugins/upload, we redirect them in a flow with a progress bar that eventually leads to a thank you page as depicted in oU6nEIeiKSg8CayhEtgR6b-fi-6559%3A46946 . This new flow also resolves #56792 with not .org plugins being uploaded.

Notices:

• I haven’t yet moved the domain warning in a modal
• Illustrations might not be final
• Thank you page alterations (like background change) will need to part of another PR

Testing instructions

Before
https://user-images.githubusercontent.com/12430020/137928110-f9661a41-6fd9-4001-b7ee-b1ae599997ec.mp4

After

With Atomic transfer

• On a Business Simple Site
• Go to /plugins/upload/:site
• Upload a plugin (the site will start transferring on the background)
• After successful installation you should land to thank you page and the plugin should be active

Without Atomic transfer

• On a Business Atomic or Ecommerce Site
• Go to /plugins/upload/:site
• Upload a plugin (the site will start transferring on the background)
• After successful installation you should land to thank you page and the plugin should be active

With Plugin already existing

• Try repeating the above steps after successfully installing the same plugin
• You should get an error page

Fixes #56792