[web] introduced thread-picker-modal.react

Summary:
introduced thread-picker-modal.react component. This diff just introduces the component, but it is not used anywhere at the moment

Next steps:

1. Implement ThreadPickerModal component
2. Clean up/get rid of old ThreadPicker component

Linear Issue: ENG-1848

Design: ENG-1778

Test Plan:
Please see the video to see how the adding calendar event interaction now works with this new component.

To test interaction on local machine:

Replace this.setState({ pickerOpen: true }); on line 213 of web/calendar/day.react.js

this.props.pushModal(
  <ThreadPickerModal
    name="Chats"
    onClose={this.props.popModal}
    createNewEntry={this.createNewEntry}
  />,
);

import ThreadPickerModal with import ThreadPickerModal from '../modals/threads/thread-picker-modal.react';

And add the popModal function by adding this code in the ConnectedDay component and Day component props:

Reviewers: atul, abosh, rohan

Reviewed By: atul

Subscribers: ashoat, tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D5317