Handle client response to initial notifications encrypted message request

Summary: This differential handles client response to initial notifications encrypted message request by creating new olm session for notifs.

Test Plan:

1. Log out.
2. Comment the code in log in panel that creates olm session for notifs.
3. Log in.
4. Query select * from olm_sessions and ensure there is a new entry for notifs

Reviewers: tomek, atul, bartek

Reviewed By: tomek, atul

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D7860