[web] replaced ThreadPicker with ThreadPicker modal

Summary:
replaced ThreadPicker component with ThreadPickerModal component.

Depends on D5317 D5239 D5289 D5230

Linear Issue: ENG-1848

Design: ENG-1778

Test Plan:
Please view the video to see how the new adding calendar event user flow works now:

Reviewers: atul, rohan, abosh

Reviewed By: atul

Subscribers: ashoat, tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D5318