Add UI for creating polls (#10184)

* Add actions and reducers for polls

* Add poll button

* Disable media upload if poll enabled

* Add poll form

* Make delete & redraft work with polls

app/javascript/mastodon/features/compose/containers/upload_button_container.js

@@ -4,6 +4,7 @@ import { uploadCompose } from '../../../actions/compose';
 
 const mapStateToProps = state => ({
   disabled: state.getIn(['compose', 'is_uploading']) || (state.getIn(['compose', 'media_attachments']).size > 3 || state.getIn(['compose', 'media_attachments']).some(m => m.get('type') === 'video')),
+  unavailable: state.getIn(['compose', 'poll']) !== null,
   resetFileKey: state.getIn(['compose', 'resetFileKey']),
 });