Issue: Importer Times Out

Even though the importer disables PHP timeout, some server configurations force a timeout if PHP doesn't produce output after a set time period anyway. Since this configuration would exist outside of PHP, there is no way to disable it from within a PHP script. Instead, the importer (and ideally install/uprade) should be refactored to allow for discovering the number of items per page dynamically -- that is, when too much time has gone by, update the next-offset based on how many items have already been processed, rather than assuming we will have enough time to reach, say 500.

In case the importer still manages to time-out (i.e. that last item we let go through takes 30 seconds by itself unexpectedly), provide a way to recover not only the current import step, but also the current batch, when clicking "Continue" on another attempt.