[web] implemented search feature for Thread Picker

Summary:
Implemented the logic for the search feature for the ThreadPicker component. There is no UI for this diff due to Search breaking the app since the focusing/blurring of the picker component prevents input in Search

ENG-1847

Depends on D5230

Test Plan:
To test the search feature, please add and modify the following blocks of code:

let searchBar = null;
if (this.props.searchIndex) {
  searchBar = (
    <Search
      searchText={this.state.searchText}
      onChangeText={this.onChangeSearchText}
      placeholder="Search chats"
    />
  );
}

You will also need to import the Search component from ../components/search.react at the top of the file

Once you have the Search component being returned you can add a new entry and test that the search feature is working as expected

Reviewers: atul, abosh, rohan, tomek

Reviewed By: atul

Subscribers: ashoat, tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D5239