Use the user list from the target project in a merge request

### What does this MR do?

This MR derives the assignable users from the target project.

### Why was this MR needed?

In a merge request, the assignable users uses the list from the forked project, not the target project where the merge request will be going. For example, if I fork a project from the gitlab-org namespace into my personal namespace, make some changes, and then create a merge request, the only users I see are me. I think it makes more sense to show all the users in gitlab-org. This is a regression caused by 1c2711f7.

### What are the relevant issue numbers?

Closes #1535

See merge request !682