[web] Changed edit modal overlay opacity

Summary: Suggested in: https://linear.app/comm/issue/ENG-3539/add-overlay-to-prevent-the-user-from-changing-thread-when-in-edit-mode

Test Plan: Checked if new opacity is applied in edit mode. Checked if it is not applied in other modals.

Reviewers: michal, inka

Reviewed By: inka

Subscribers: ashoat, tomek, atul, ted

Differential Revision: https://phab.comm.dev/D7930