Due to a bug in `BoardPolicy`, users were getting back a 403 error when
trying to assign users to an assignee list and seeing "Something went
wrong while fetching assignees list". For some reason, the declarative
policy runtime was ignoring the ternary condition.

To work around the issue, we make the project board an explicit
condition check.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/9727