Stepper: Migrate hosted site flow to use built in auth

[alshakero]

Proposed Changes

This migrates the new-hosted-site flow to use built-in Stepper auth. This is much faster to load and makes all the navigations SPA-style. It also enables preloading of the following steps in the flow.

Why are these changes being made?

Mostly performance.

Testing Instructions

1. Please test the flow thoroughly around auth.
2. Try to use it while logged in.
3. Try to use it while logged out by signing up.
4. Try to use it while logged out by signing in.

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-134830

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-134830&env=jetpack

Automattic for Agencies live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-134830&env=a8c-for-agencies

[matticbot]

This PR modifies the release build for the following Calypso Apps:

For info about this notification, see here: PCYsg-OT6-p2

• notifications
• wpcom-block-editor

To test WordPress.com changes, run install-plugin.sh $pluginSlug upgradte/new-hosted-flow-user on your sandbox.

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

Sections (~8 bytes removed 📉 [gzipped])

name                  parsed_size           gzip_size
new-hosted-site-flow        -51 B  (-0.0%)       -8 B  (-0.0%)

Sections contain code specific for a given set of routes. Is downloaded and parsed only when a particular route is navigated to.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.