Subscriptions and notifications for namespaces, tasks and lists (#786)

Co-authored-by: kolaente <k@knt.li>
Reviewed-on: https://kolaente.dev/vikunja/api/pulls/786
Co-authored-by: konrad <konrad@kola-entertainments.de>
Co-committed-by: konrad <konrad@kola-entertainments.de>

pkg/db/fixtures/subscriptions.yml

@@ -0,0 +1,35 @@
+- id: 1
+  entity_type: 3 # Task
+  entity_id: 2
+  user_id: 1
+  created: 2021-02-01 15:13:12
+- id: 2
+  entity_type: 1 # Namespace
+  entity_id: 6
+  user_id: 6
+  created: 2021-02-01 15:13:12
+- id: 3
+  entity_type: 2 # List
+  entity_id: 12 # belongs to namespace 7
+  user_id: 6
+  created: 2021-02-01 15:13:12
+- id: 4
+  entity_type: 3 # Task
+  entity_id: 22 # belongs to list 13 which belongs to namespace 8
+  user_id: 6
+  created: 2021-02-01 15:13:12
+- id: 5
+  entity_type: 1 # Namespace
+  entity_id: 8
+  user_id: 6
+  created: 2021-02-01 15:13:12
+- id: 6
+  entity_type: 2 # List
+  entity_id: 13
+  user_id: 6
+  created: 2021-02-01 15:13:12
+- id: 7
+  entity_type: 3 # Task
+  entity_id: 26
+  user_id: 6
+  created: 2021-02-01 15:13:12