Milestone header: Migrate kebab menu to GlDisclosureDropdown

...ascripts/milestones/components/delete_milestone_modal.vue

...vascripts/milestones/components/more_actions_dropdown.vue

...scripts/milestones/components/promote_milestone_modal.vue

app/assets/javascripts/milestones/index.js

...sets/javascripts/milestones/init_more_actions_dropdown.js

...ets/javascripts/pages/dashboard/milestones/index/index.js

...assets/javascripts/pages/groups/milestones/index/index.js

app/assets/javascripts/pages/groups/milestones/show/index.js

...sets/javascripts/pages/projects/milestones/index/index.js

...ssets/javascripts/pages/projects/milestones/show/index.js

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

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

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

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

locale/gitlab.pot

spec/features/groups/milestone_spec.rb

spec/features/milestone_spec.rb

spec/features/milestones/user_promotes_milestone_spec.rb

...ntend/milestones/components/more_actions_dropdown_spec.js

...end/milestones/components/promote_milestone_modal_spec.js