Add wpcom to onboarding

[nightnei]

Related issues

• Fixes STU-972

Proposed Changes

With this PR we are adding wpcom as option for onboarding step

Testing Instructions

1. Open appdata-v1.json, set "onboardingCompleted": false and remove authToken if you have it
2. Open Studio
3. Assert that you see the next screen
   
4. Click on "Skip"
5. Assert that onboarding is closed and you see initial Studio's empty page for creating new site
6. Repeat 1-st step to clean Studio
7. Click on "Create a free account"
8. Assert that you were landed to the appropriate wpcom page
9. Next, click on Log in and proceed
10. Assert that you landed to Studio and you see again Studio's empty page

[sejas]

Yay!, Love the PRs that remove so much code, now the flows are even simpler.
We can tackle the exact copy of the onboarding screen as part of STU-1058
I drafted a PR to replace the empty screen with the Add site options flow here #2149

oauth-onboarding.mp4

[gavande1]

I have tested this and it works as described. LGTM 👍

[github-actions]

📊 Performance Test Results

Comparing f579b7b vs trunk

site-editor

Metric	trunk	f579b7b	Diff	Change
load	17016.00 ms	15958.00 ms	-1058.00 ms	🟢 -6.2%

site-startup

Metric	trunk	f579b7b	Diff	Change
siteCreation	19340.00 ms	19527.00 ms	+187.00 ms	🔴 1.0%
siteStartup	6978.00 ms	7945.00 ms	+967.00 ms	🔴 13.9%

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Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change