Add tracker_date_reminder tables and examples for tests
First test to migrate reminders
assert correctness of ugroups
assert that a reminder is created for each mail
notifications may start after the date
no reminders migrated when the list of ugroups is empty
add missing file
refactoring to decrease indentation level
Merge branch '2862_migrate_tracker_date_reminder' into stable

* 2862_migrate_tracker_date_reminder:
  refactoring to decrease indentation level
  add missing file
  no reminders migrated when the list of ugroups is empty
  notifications may start after the date
  assert that a reminder is created for each mail
  assert correctness of ugroups
  First test to migrate reminders
  Add tracker_date_reminder tables and examples for tests
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