Add advanced search dropdown to React editor

Allows a dropdown panel to open that includes all the fields of multiple-field search, including the filter toggles.

1. Values entered in multi-field search should be reflected in the values shown against each field on the dropdown panel.
2. Typing a value in a field of the dropdown panel should update the multi-field search string to include what is typed (while it is typed).

Technical note

• component is already implemented in storybook
• date inputs are strings for now (later they will be calendar inputs)
• needs a new API endpoint
  • move the GWT handler code to a service
  • call the service from the GWT handler
  • call the service from the new REST API endpoint
• API endpoint will re-use the code from the GWT action handler
• needs to filter the textflow list on the editor app