Merge branch 'async-milestone-tabs' into 'master'

app/assets/javascripts/milestone.js

app/controllers/concerns/milestone_actions.rb

app/controllers/groups/milestones_controller.rb

app/controllers/projects/milestones_controller.rb

app/helpers/milestones_helper.rb

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

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

changelogs/unreleased/async-milestone-tabs.yml

config/routes/group.rb

config/routes/project.rb

features/group/milestones.feature

features/project/milestone.feature

features/steps/group/milestones.rb

features/steps/project/project_milestone.rb

spec/controllers/groups/milestones_controller_spec.rb

spec/controllers/projects/milestones_controller_spec.rb

spec/features/milestones/milestones_spec.rb

spec/support/milestone_tabs_examples.rb