Add feature flag for polls and placeholder dialog

We're anticipating development of polls soon, so for efficiency reasons we're getting the labs flag in early. If something goes wrong with the plan, this whole change will be reverted.

src/components/views/rooms/MessageComposer.tsx

@@ -52,6 +52,8 @@ import EditorModel from "../../../editor/model";
 import EmojiPicker from '../emojipicker/EmojiPicker';
 import MemberStatusMessageAvatar from "../avatars/MemberStatusMessageAvatar";
 import UIStore, { UI_EVENTS } from '../../../stores/UIStore';
+import Modal from "../../../Modal";
+import InfoDialog from "../dialogs/InfoDialog";
 
 let instanceCount = 0;
 const NARROW_MODE_BREAKPOINT = 500;
@@ -193,6 +195,31 @@ class UploadButton extends React.Component<IUploadButtonProps> {
     }
 }
 
+// TODO: [polls] Make this component actually do something
+class PollButton extends React.PureComponent {
+    private onCreateClick = () => {
+        Modal.createTrackedDialog('Polls', 'Not Yet Implemented', InfoDialog, {
+            // XXX: Deliberately not translated given this dialog is meant to be replaced and we don't
+            // want to clutter the language files with short-lived strings.
+            title: "Polls are currently in development",
+            description: "" +
+                "Thanks for testing polls! We haven't quite gotten a chance to write the feature yet " +
+                "though. Check back later for updates.",
+            hasCloseButton: true,
+        });
+    };
+
+    render() {
+        return (
+            <AccessibleTooltipButton
+                className="mx_MessageComposer_button mx_MessageComposer_poll"
+                onClick={this.onCreateClick}
+                title={_t('Create poll')}
+            />
+        );
+    }
+}
+
 interface IProps {
     room: Room;
     resizeNotifier: ResizeNotifier;
@@ -432,6 +459,11 @@ export default class MessageComposer extends React.Component<IProps, IState> {
     private renderButtons(menuPosition): JSX.Element | JSX.Element[] {
         const buttons: JSX.Element[] = [];
         if (!this.state.haveRecording) {
+            if (SettingsStore.getValue("feature_polls")) {
+                buttons.push(
+                    <PollButton key="polls" />
+                );
+            }
             buttons.push(
                 <UploadButton key="controls_upload" roomId={this.props.room.roomId} />,
             );