feat: Add relative Reminders (#1427)

Partially resolves #1416

Co-authored-by: ce72 <christoph.ernst72@googlemail.com>
Reviewed-on: https://kolaente.dev/vikunja/api/pulls/1427
Reviewed-by: konrad <k@knt.li>
Co-authored-by: cernst <ce72@noreply.kolaente.de>
Co-committed-by: cernst <ce72@noreply.kolaente.de>

pkg/modules/migration/microsoft-todo/microsoft_todo_test.go

@@ -141,8 +141,10 @@ func TestConverting(t *testing.T) {
 						{
 							Task: models.Task{
 								Title: "Task 5",
-								Reminders: []time.Time{
-									testtimeTime,
+								Reminders: []*models.TaskReminder{
+									{
+										Reminder: testtimeTime,
+									},
 								},
 							},
 						},