Milestone tabs are not hidden when issues / MRs are disabled or for project members only

When issues are disabled, the issues tab used to be hidden but was reverted in !18818 (comment 234143965) because of a bug. For merge requests, the tab should also be hidden when the repository feature is disabled or if it is set to "Project members only" and the user isn't a member.

There is no security issue here because when the feature is disabled or the user doesn't have access, it just shows an empty list.

Note that we are in the process of redesigning the milestones page so we may want to defer this as this might not be needed with the redesign.