do not check 'remember' log in by default

Finally end the discussion of #3918.

Still TO DO:

* [ ] if the "remember" box is checked, the setting in config.php ```session_lifetime``` does not seem to work. (like @enoch85 said in #6866)
* [ ] if "remember" is checked, it should also remain checked when I logged out.

Can you help there @PVince81?