[web] Notifications dialog - disable background option

Summary:
ENG-1729
Disables background option in notifications dialog for sidebars, because they can't be moved between tabs anyway.

Test Plan: Check that nothing changed for non-sidebars. Check that there is no radio button for Background options and it can't be selected for sidebars.

Reviewers: tomek, atul, abosh

Reviewed By: atul

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D5280