minor fix in reminders module.
added todo for description.
use boolean instead of negation operator.
use Boolean in update_sent_forms filter.
used getClinicPhone helper in add_regimes filter.
fixed bug in reminders schedule.

reminders were getting added even when configuration was not valid
(missing form). so added isConfigValid() to reminders schedule that
checks for form, messages and either text or cron config.
minor fixes to reminders tests.
fixed bug in due_tasks with endkey/startkey

use new date format call `.toISOString()`