[web] [feat] [ENG-991] add modal prompt

Summary:
add modal to web to prompt user about promote thread.

https://linear.app/comm/issue/ENG-1044/fix-regression-introduced-in-eng-972-justify-modal-header-content there's an issue with the modal header documented here. There's a little too much padding here.

Test Plan: click promote thread option. Test promote functionality and cancel promote functionality

Reviewers: atul, palys-swm, def-au1t, yayabosh, ashoat

Reviewed By: palys-swm, def-au1t, ashoat

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

Differential Revision: https://phabricator.ashoat.com/D3809