Merge trains link shows when user is not logged in

Problem

The merge trains link shows on the merge requests page when a user is not logged in. Once you click the link it takes you to a 404 page.

Implementation Guide

In ee/app/views/projects/merge_requests/_merge_trains_button.html.haml update the conditional to - if merge_trains_available?(@project) && @current_user && can?(@current_user, :read_merge_train, @project) and add the appropriate test coverage.