Honour issue and merge request visibility in their respective finders

This MR fixes a security issue with the IssuesFinder and MergeRequestFinder where they would return items the user did not have permission to see. This was most visible on the issue and merge requests page for a group containing projects that had set their issues or merge requests to "private".

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/22481

See merge request !2000
Signed-off-by: Rémy Coutable <remy@rymai.me>