Remove token and user id keys from session and local storage on logout

Remove 'accessTokenId' and 'currentUserId' keys from session and local storage on logout.
Fixes #61 
> I've kept those lines and, in fact, introduced a completely different it(...) clause in tests without affecting any of the previous tests.

Perfect!

> I'm sorry, I'm not sure what you mean by ... If you keep an empty value, the test will fail if it checks wrong item names. Please elaborate.

Imagine a future developer changes the item names, e.g. rename `propsPrefix` from `$LoopBack$` to `LoopBack:`. If the test is checking that `$LoopBack$currentUserId` is not defined after `logout`, it will always pass, because `$LoopBack$currentUserId` is no longer set by `login` in the first place.