Allow to assign a MR to any person with access to the original branch

Summary

It is impossible to assign an MR to some of its authors.

Steps to reproduce

1. Create a repository A1 with members X, Y.
2. Clone the repository A1 into repository A2 with members Z, T
3. Let Z & T jointly develop a feature branch in A2.
4. Let Z submit a merge request to A1.
5. Observe the list of people to whom the MR can be assigned.

Expected behavior

The MR can be assigned to X, Y, Z, T.

Actual behavior

The MR can be assigned to X, Y, Z.