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/db/fixtures/task_reminders.yml

@@ -6,6 +6,8 @@
   task_id: 27
   reminder: 2018-12-01 01:13:44
   created: 2018-12-01 01:12:04
+  relative_to: 'start_date'
+  relative_period: -3600
 - id: 3
   task_id: 2
   reminder: 2018-12-01 01:13:44

pkg/db/fixtures/tasks.yml

@@ -244,7 +244,7 @@
   created: 2018-12-01 01:12:04
   updated: 2018-12-01 01:12:04
 - id: 27
-  title: 'task #27 with reminders'
+  title: 'task #27 with reminders and start_date'
   done: false
   created_by_id: 1
   project_id: 1
@@ -252,6 +252,7 @@
   bucket_id: 1
   created: 2018-12-01 01:12:04
   updated: 2018-12-01 01:12:04
+  start_date: 2018-11-30 22:25:24
 - id: 28
   title: 'task #28 with repeat after'
   done: false