- [x] Move `CreateRepositoryByExample` to service layer
- [x] Move `AddCollabrator` to service layer
- [x] Add a new parameter for `AddCollabrator` so that changing mode
immediately after that will become unnecessary.
The method can't be called with an outer transaction because if the user
is not a collaborator the outer transaction will be rolled back even if
the inner transaction uses the no-error path.
`has == 0` leads to `return nil` which cancels the transaction. A
standalone call of this method does nothing but if used with an outer
transaction, that will be canceled.
