[web] cleanup UnsavedChangesModal

Summary:
This diff cleans up the onse off styles in the UnsavedChangesModal and updates the modal to follow the conventions of the new modal api

Linear task: https://linear.app/comm/issue/ENG-5943/extendmodify-the-modal-props-api-to-follow-new-modal-designs

Depends on D11024

Test Plan:
Please see the screenshot below

Reviewers: atul, inka

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11025