Fix credentials refreshment in Google OAuth2

[adekbadek]

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Changes proposed in this Pull Request:

Fixes an issue with Google OAuth, where even though refresh token was stored, it was not used to refresh credentials.
Two scenarios have to be handled here - if a user authorises the app for the first time, they will receive a refresh token (allowing them to connect after the access token expires). Otherwise, they will receive only an access token, which will expire after a certain amount to time.

How to test the changes in this Pull Request:

1. Revoke access to the OAuth2 app on your Google permissions page and remove credentials (_newspack_google_oauth user meta) from the DB
2. Complete the oauth flow (Add Google OAuth2 authentication flow  #935) to ensure the happy path is covered
3. Remove credentials from the DB, observe the oauth flow can be completed again
4. Revoke access in Google permissions page, observe the oauth flow can be completed again
5. Invalidate the credentials by editing the user meta and changing the values of tokens to gibberish
6. Observe the oauth flow can be completed again
7. Revoke access, complete the flow again (to ensure you'll get a refresh token)
8. Come back after an hour or more - observe the dashboard still displays the authenticated state

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?

[dkoo]

Works as described—still authenticated after a whole day.

[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 📦🚀