Improve method names and add more specs

app/models/concerns/milestoneish.rb

app/models/dashboard_milestone.rb

app/models/group_milestone.rb

app/models/milestone.rb

app/serializers/issuable_entity.rb

app/views/shared/milestones/_sidebar.html.haml

app/views/shared/milestones/_tabs.html.haml

app/views/shared/milestones/_top.html.haml

spec/models/concerns/milestoneish_spec.rb

spec/models/group_milestone_spec.rb

spec/requests/api/milestones_spec.rb