make the facebook personal feed page back in time

This is done by saving the oldest item on the page and passing it back
as 'until' on the next iteration. 'Since' is always passed in so we
page back until the last time we synced, so existing profiles will
need their configs reset.
clean up a few things and ensure we can page forward
only page back if we got a new entry
update tests to not use old next variable
remove extra comments
small changes for clarity
make sure we always save since
Merge branch 'facebook-feed-paging' into peas-in-a