Only set dav user after login.

Setting the credentials of the account inside the auth widget is not a
good idea as that will destroy the previous credentials object which
may wait for a signal to be emitted by the credentials dialog that
was created by the credentials that are going to be deleted. Uff.

It should be enough to set the dav user only after login because the
dav user will never change.

See also the discussion here
https://github.com/nextcloud/desktop/issues/3677#issuecomment-907976839



Fixes #3677

Signed-off-by: Felix Weilbach <felix.weilbach@nextcloud.com>