Update the request reminder email tasks to make them simpler to test

* return the request/record IDs for which reminders will be sent out, to
  enable simpler asserts in the tests
* handle the case that request topic's PID cannot be resolved - this is
  likely when the topic has not been published yet
* include "today" in the "accepted review" reminder, as too new requests
  will be filtered out anyway and are not expected to add significant
  load