[web] Update thread promotion modal on web
Summary:
This diff updates the thread promotion modal on web, following the Design task.
- Updated modal_body padding to 24px 32px 32px 32px
- Make the button the usual purple color
- Remove the line break between the text
Design Task: https://linear.app/comm/issue/DES-14/thread-promote-button-modal-looks-scary
Engineering Task: https://linear.app/comm/issue/ENG-2410/update-thread-promotion-modal-on-web
Test Plan:
See screenshots below
Before:
After:
Reviewers: atul, ginsu, ashoat
Reviewed By: ashoat
Subscribers: ashoat, tomek
Differential Revision: https://phab.comm.dev/D5874