Introduces a wrapper method for easily disabling the oAuth way.

[andizer]

Summary

This PR can be summarized in the following changelog entry:

• N/A -

Relevant technical choices:

• Made a wrapper for easily disabling the oauth feature. This returns false at the moment. Should be returning the commented value when oAuth is implemented.
• Also commented out the oAuth loader, to make sure the oAuth feature isn't loaded.

Test instructions

This PR can be tested by following these steps:

Previously the add-ons update process would go through the license manager. This got rewritten with the MyYoast connection flow. Unfortunately, this is more complicated and should get reverted a bit for now.

Therefore, we should test if the detecting and actual updating of the add-ons functions as before.

• Install an older version of an add-on (see releases tab on GitHub).
• Ensure that it still detects that there is a newer version available (it should see this on the plugins page, but there is also a Check Again button in the Dashboard - Updates page).
• Ensure that updating the add-on works.

UI changes

• This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Quality assurance

• I have tested this code to the best of my abilities
• I have added unittests to verify the code works as intended

Fixes #

[afercia]

CR 🚧 Locally, I get this with PHPUnit 6.5.8:

There was 1 error:

1) Yoast\Tests\UnitTests\Oauth\Oauth_Client_Test::test_get_provider
Error: Call to undefined function YoastSEO_Vendor\GuzzleHttp\choose_handler()

[afercia]

I've marked Yoast\Tests\UnitTests\Oauth\Oauth_Client_Test::test_get_provider as test to skip for now. The test failed because require_once WPSEO_PATH . 'src/loaders/oauth.php'; is now commented thus test_get_provider is undefined.

[afercia]

CR 2 👍

[igorschoester]

Acceptance ✅