Re-enable auto-reconnect, after fixing logout.

Summary:

• Disconnect websocket in ~DDPClient to avoid auto-reconnect
• Make logout actually work by returning true in canStart() (obvious typo)

Also, don't clear the password on logout. I want to be able to log in again
without having to type the password again. No idea why this clear() was
there.

Cleanups: remove unused signal logoutDone() and unimplemented method DDPClient::logOut

Test Plan:
Logout stays on login page, with password filled in.
Disconnecting from WIFI for a few minutes and then connecting again leads to the server
throwing us out, and ruqola reconnects.

Reviewers: mlaurent, kfunk

Differential Revision: https://phabricator.kde.org/D27387