Add Google OAuth2 authentication flow

[adekbadek]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Adds an OAuth2 authentication flow, as a preparation for the upcoming GAM integration.
Currently, Site Kit's authorisation is reused to access user's GA data. Site Kit does not support the scopes necessary to access GAM data, so Newspack has to roll its own authorisation for this purpose.

UI: I have made it very lightweight purposefully, since this PR is quite large as it is. The UI should probably live in some kind of settings view. cc @thomasguillot

How to test the changes in this Pull Request:

1. Nothing should change unless the necessary environment variables are present, so have a look at the Dashboard, Campaigns Analytics, and Analytics wizard - everything should work as before
2. Create a Google OAuth2 app. Set <test-site->/wp-admin/admin.php?page=newspack (e.g. https://newspack.test/wp-admin/admin.php?page=newspack) as an Authorized redirect URI.
3. Set client ID and secret of the app as NEWSPACK_GOOGLE_OAUTH_CLIENT_ID and NEWSPACK_GOOGLE_OAUTH_CLIENT_SECRET environment variables in wp-config.php
4. Visit the Dashboard, observe a new section above the wizard links - with an "Authorise" button
5. Click the button and complete the authorisation flow - observe a success message and information "Authorized Google as " in the end
6. This was the happy path, now let's test how it can break. Remove the _newspack_google_oauth user meta from the DB
7. Reload the Dasboard, observe initial (pre-authorisation) state. Complete the authorisation from again.
8. Visit your Google account's permissions screen and revoke permissions for the app
9. Again, observe the pre-authorisation state and complete the flow again.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[thomasguillot]

@adekbadek I moved the OAuth2 section to the bottom of the dashboard.

Not quite sure but goToAuthPage didn't seem to work... not quite sure why 🤔

UI: I have made it very lightweight purposefully, since this PR is quite large as it is. The UI should probably live in some kind of settings view

Do you mean the actual connection, or the client ID/secret?

[adekbadek]

UI: I have made it very lightweight purposefully, since this PR is quite large as it is. The UI should probably live in some kind of settings view

Do you mean the actual connection, or the client ID/secret?

The connection - it's better after your changes, but still it's an odd item among links to wizards.

[adekbadek]

Not quite sure but goToAuthPage didn't seem to work... not quite sure why 🤔

What happens after you click on the authorisation dashboard item?

[thomasguillot]

What happens after you click on the authorisation dashboard item?

Absolutely nothing

[thomasguillot]

The connection - it's better after your changes, but still it's an odd item among links to wizards.

Oh ok I see.
What if we replicate something like the integrations step from the onboarding? Where a user could see Jetpack, Site Kit, Mailchimp, and Google OAuth statuses

[adekbadek]

What if we replicate something like the integrations step from the onboarding?

Definitely, but that's on top of the dashboard/post-onboarding UI.

[dkoo]

Really like the consolidation of the Google Services authorizations into a single class, and the DRY-ability(?) it enables. I have a few nits below but nothing blocking, so 👍

[matticbot]

🎉 This PR is included in version 1.37.0-alpha.1 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀

[matticbot]

🎉 This PR is included in version 1.37.0 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀