Simplify markdown form header

.../javascripts/content_editor/components/content_editor.vue

...ascripts/content_editor/components/formatting_toolbar.vue

...ascripts/content_editor/services/create_content_editor.js

...issues/show/components/incidents/timeline_events_form.vue

...ets/javascripts/notes/components/comment_field_layout.vue

...sets/javascripts/notes/components/noteable_discussion.vue

...s/vue_shared/components/markdown/editor_mode_switcher.vue

...sets/javascripts/vue_shared/components/markdown/field.vue

...ets/javascripts/vue_shared/components/markdown/header.vue

...cripts/vue_shared/components/markdown/markdown_editor.vue

...ts/javascripts/vue_shared/components/markdown/toolbar.vue

app/assets/stylesheets/components/content_editor.scss

app/assets/stylesheets/framework/markdown_area.scss

app/assets/stylesheets/pages/note_form.scss

app/views/shared/_md_preview.html.haml

app/views/shared/notes/_hints.html.haml

...end/content_editor/services/create_content_editor_spec.js

...nents/__snapshots__/snippet_description_edit_spec.js.snap