Repository transfer has to be confirmed, if user can not create repo for new owner (#14792)

* make repo as "pending transfer" if on transfer start doer has no right to create repo in new destination

* if new pending transfer ocured, create UI & Mail notifications

models/error.go

@@ -757,6 +757,40 @@ func (err ErrRepoNotExist) Error() string {
 		err.ID, err.UID, err.OwnerName, err.Name)
 }
 
+// ErrNoPendingRepoTransfer is an error type for repositories without a pending
+// transfer request
+type ErrNoPendingRepoTransfer struct {
+	RepoID int64
+}
+
+func (e ErrNoPendingRepoTransfer) Error() string {
+	return fmt.Sprintf("repository doesn't have a pending transfer [repo_id: %d]", e.RepoID)
+}
+
+// IsErrNoPendingTransfer is an error type when a repository has no pending
+// transfers
+func IsErrNoPendingTransfer(err error) bool {
+	_, ok := err.(ErrNoPendingRepoTransfer)
+	return ok
+}
+
+// ErrRepoTransferInProgress represents the state of a repository that has an
+// ongoing transfer
+type ErrRepoTransferInProgress struct {
+	Uname string
+	Name  string
+}
+
+// IsErrRepoTransferInProgress checks if an error is a ErrRepoTransferInProgress.
+func IsErrRepoTransferInProgress(err error) bool {
+	_, ok := err.(ErrRepoTransferInProgress)
+	return ok
+}
+
+func (err ErrRepoTransferInProgress) Error() string {
+	return fmt.Sprintf("repository is already being transferred [uname: %s, name: %s]", err.Uname, err.Name)
+}
+
 // ErrRepoAlreadyExist represents a "RepoAlreadyExist" kind of error.
 type ErrRepoAlreadyExist struct {
 	Uname string