Basic username autocomplete for text area

app/assets/javascripts/components/actions/compose.jsx

...cripts/components/features/ui/components/compose_form.jsx

...ripts/components/features/ui/components/upload_button.jsx

...ponents/features/ui/containers/compose_form_container.jsx

app/assets/javascripts/components/reducers/compose.jsx

app/assets/stylesheets/components.scss

package.json

yarn.lock