Delete related PullAutoMerge and ReviewState on User/Repo Deletion (#19649)

* delete pullautomerges on repo/user deletion
* delete reviewstates on repo/user deletion
* optimize automerhe code
* add index to reviewstate

models/db/error.go

@@ -42,3 +42,18 @@ func IsErrSSHDisabled(err error) bool {
 func (err ErrSSHDisabled) Error() string {
 	return "SSH is disabled"
 }
+
+// ErrNotExist represents a non-exist error.
+type ErrNotExist struct {
+	ID int64
+}
+
+// IsErrNotExist checks if an error is an ErrNotExist
+func IsErrNotExist(err error) bool {
+	_, ok := err.(ErrNotExist)
+	return ok
+}
+
+func (err ErrNotExist) Error() string {
+	return fmt.Sprintf("record does not exist [id: %d]", err.ID)
+}