[web] Check if notifications are supported

Summary: Fix for ENG-3340. Mobile browsers don't all support the web Notifications API. We should add a check for that.

Test Plan: Check in safari on iOS emulator. Before the changes, website displays the error. After the diff it works fine.

Reviewers: kamil, inka, kuba

Reviewed By: kamil

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7329