[web] Change ConfirmLeaveThreadModal size to fit-content

Summary:
After Modal redesign, the modal size must be set to fit-content to display it with correct size.

Before:

Test Plan: Try to leave the thread in web app - the modal size should now be correct

Reviewers: palys-swm, benschac, atul

Reviewed By: palys-swm, atul

Subscribers: ashoat, palys-swm, Adrian, atul, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3836