Fix token page URL
Summary:
It seems like Google changed the URL of the token page.
Maybe we should use QString::startsWith, to be futureproof?
BUG: 388483
Test Plan:
- Login in KOrganizer and KMail works again
- All tests pass (though i think there is no oauth2 test)
Reviewers: dvratil, mlaurent
Reviewed By: mlaurent
Differential Revision: https://phabricator.kde.org/D9843