MM-31875 soft delete reactions (#16654)

- soft delete reaction by setting new field Reactions.DeleteAt to non-zero.
- include new field Reactions.UpdateAt

i18n/en.json

@@ -7946,6 +7946,10 @@
     "id": "model.reaction.is_valid.post_id.app_error",
     "translation": "Invalid post id."
   },
+  {
+    "id": "model.reaction.is_valid.update_at.app_error",
+    "translation": "Update at must be a valid time."
+  },
   {
     "id": "model.reaction.is_valid.user_id.app_error",
     "translation": "Invalid user id."