Remove milestone component from sidebar if project/group has no milestones
The milestone component is taking up space in the issue sidebar and may not be necessary for all users:
Relevant research (ux-research#134 (comment 177072293)):
Epics should be removed for projects that don't use them (for example in free accounts the feature is locked and takes space for no reason in the sidebar). Milestones could probably be removed too for repositories that don't have any