Merge branch 'winh-epics-tree-create-issue-button' into 'master'

...assets/javascripts/vue_shared/components/split_button.vue

app/assets/stylesheets/framework/dropdowns.scss

...ripts/related_items_tree/components/create_issue_form.vue

...ated_items_tree/components/issue_actions_split_button.vue

.../related_items_tree/components/related_items_tree_app.vue

...lated_items_tree/components/related_items_tree_header.vue

ee/app/controllers/groups/epics_controller.rb

ee/spec/features/epics/epic_issues_spec.rb

...ated_items_tree/components/related_items_tree_app_spec.js

...d_items_tree/components/related_items_tree_header_spec.js

locale/gitlab.pot

...shared/components/__snapshots__/split_button_spec.js.snap

spec/frontend/vue_shared/components/split_button_spec.js