[MM-53291] Data retention improvements (#24253)

* adding new migration for RetentionIdsForDeletion, changing logic for deleting orphaned reactions. Updating delete user and channel endpoints to remove respective reactions

server/public/model/data_retention_policy.go

@@ -84,3 +84,15 @@ type RetentionPolicyCursor struct {
 	TeamPoliciesDone    bool
 	GlobalPoliciesDone  bool
 }
+
+type RetentionIdsForDeletion struct {
+	Id        string
+	TableName string
+	Ids       []string
+}
+
+func (r *RetentionIdsForDeletion) PreSave() {
+	if r.Id == "" {
+		r.Id = NewId()
+	}
+}