Encrypt notifications for web clients supporting decryption

Summary: This differential encrypts notifications for sufficiently new web clients.

Test Plan:

1. Change FUTURE_CODE_VERSION flag to 0.
2. Open web client.
3. Add console.log() to log data received in service-worker and early return to skip this notification.
4. Send web notification.
5. Examine service-worker logs - ensure that encrypted payload is visible.
6. Using mariadb console ensure that version field of olm_session associated with cookie_id of current web client increments with each notication.

Reviewers: michal, kamil

Reviewed By: michal

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9620