Include Epics via helper in Group Milestone homepage

Summary

For the updates to Group Milestone page as proposed in #425088 (closed) where we need to show Epics alongside other items, we need to ensure that epics are included in the issuables helper that's used in app/views/shared/milestones/_issuables.html.haml#L18

Note that since Epics are EE only, we need to make sure that we create an EE version of app/helpers/milestones_helper.rb which starts including Epics along with Issues which have a milestone assigned, this will then be consumed in HAML to show the information on Milestone page at group level.