Patch from Christian Dywan

Prefer logging in via the saved auth token

Summary:
This in particular avoids re-authenticating with a 2FA code every time Ruqola is opened (see bug 409212).

BUG: 409213

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: mlaurent

Differential Revision: https://phabricator.kde.org/D22112