UI for uploading media attachments (and cancelling them)

app/assets/images/void.png

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

app/assets/javascripts/components/components/button.jsx

...assets/javascripts/components/components/compose_form.jsx

app/assets/javascripts/components/components/frontend.jsx

...ssets/javascripts/components/components/upload_button.jsx

app/assets/javascripts/components/components/upload_form.jsx

...vascripts/components/containers/upload_form_container.jsx

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

app/assets/stylesheets/components.scss

app/services/post_status_service.rb

app/views/api/media/create.rabl