Add all users of a project to another project

Description

Soon you'll be able to add members of a group to another project: https://gitlab.com/gitlab-org/gitlab-ce/issues/19868

It would make user management very easy if you could do the same with projects.

Proposal

Do the same as https://gitlab.com/gitlab-org/gitlab-ce/issues/19868, but for project members:

Links / references

• Part of larger issue #19860 (moved)