The implementation is now extracted into an interface.
That way it is easier to replace the concrete implementation for cookie
handling
with a custom one.
Relates: #21
Fix broken cookie path within middleware. For some reason we used the
`/typo3/` path while storing cookies server side. But we used `/` in
JavaScript. That didn't play together and was fixed to always be `/` for
now, but it should be configurable in general. The fix revealed that the
detection of whether to store a cookie was broken, which was fixed
within the corresponding service.
Furthermore the dates how long the cookie should be stored was
different. We now always use 7 days.
