[web] focus search input for thread picker modal

Summary:
The search input in the Thread Picker Modal now focuses whenever the thread picker opens up. This saves the user a click and is what we do on native

Linear Task: ENG-2051

Test Plan:
Please watch the demos to see the changes I made. Please note that only the Search component in the Thread Picker Modal has this focus change, and other places that use the Search components will remain the same

Before:

After:

Reviewers: atul, rohan, ashoat

Reviewed By: atul, rohan, ashoat

Subscribers: ashoat, tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D5471