Diff threads: Align toggle function with regular threads

...s/javascripts/notes/components/diff_discussion_header.vue

app/assets/javascripts/notes/components/discussion_notes.vue

app/assets/javascripts/notes/components/note_header.vue

...ts/javascripts/notes/components/toggle_replies_widget.vue

app/assets/stylesheets/pages/notes.scss

locale/gitlab.pot

.../user_resolves_diff_notes_and_discussions_resolve_spec.rb

...es/merge_request/user_sees_discussions_navigation_spec.rb

.../frontend/notes/components/diff_discussion_header_spec.js

spec/frontend/notes/components/note_header_spec.js

spec/frontend/notes/components/toggle_replies_widget_spec.js

.../notes/__snapshots__/work_item_note_replying_spec.js.snap