check remember log in by default, ref #3918

Because there’s more use-cases for remembering than not to. If you don’t want it to be remembered, you can just uncheck it.

See also https://github.com/owncloud/core/issues/3918?source=cc for discussion if the checkbox is needed at all.

Please check it @owncloud/designers @LukasReschke @DeepDiver1975