Remove the possibility of the user getting stuck when managing open-source Qt

Current behaviour
See https://forum.qt.io/topic/70767/refused-access-to-my-qt-maintainancetool

While trying to upgrade, a user got stuck with the message...

No valid license available in your Qt Account for this host platform.
Please renew your license(s) from Qt Account.

...but they did not have any license to begin with, so they couldn't possibly renew.

The solution was to clear the email field. This then allowed the user to click "Skip".

This is not intuitive.

Suggested behaviour
Even if credentials have been entered, don't disable the "Skip" button. Instead, let it pop-up a confirmation dialog that gives the user the option to either proceed with their entered credentials, or to skip the login.