Milestone sidebar widget

...s/javascripts/boards/components/board_content_sidebar.vue

...avascripts/sidebar/components/sidebar_dropdown_widget.vue

app/assets/javascripts/sidebar/constants.js

...ts/javascripts/sidebar/queries/milestone.fragment.graphql

.../sidebar/queries/project_issue_milestone.mutation.graphql

...pts/sidebar/queries/project_issue_milestone.query.graphql

...ascripts/sidebar/queries/project_milestones.query.graphql

...avascripts/sidebar/components/sidebar_dropdown_widget.vue

ee/app/assets/javascripts/sidebar/constants.js

...mponents/__snapshots__/board_content_sidebar_spec.js.snap

.../frontend/boards/components/board_content_sidebar_spec.js

spec/features/boards/sidebar_milestones_spec.rb

.../frontend/boards/components/board_content_sidebar_spec.js

...ontend/sidebar/components/sidebar_dropdown_widget_spec.js

spec/frontend/sidebar/mock_data.js