[web] First pass at styling CantLeaveThreadModal
Summary:
Started with styles borrowed from thread-settings-modal.css and made some tweaks so it looks decent.
Before:
After:
Test Plan:
- Replaced InvalidUploadModal in InputStateContainer since CantLeaveThreadModal is currently unused in codebase.
- Dragged SVG into message list
- CantLeaveThreadModal looked as expected
Reviewers: def-au1t, palys-swm
Reviewed By: palys-swm
Subscribers: ashoat, Adrian, karol-bisztyga, benschac, yayabosh
Differential Revision: https://phabricator.ashoat.com/D3708