Establish olm sessions for notifs on both sides during log-in

Summary: This differential implements establishment of olm sessions for e2e notifs on both sides during log in.

Test Plan: Log in to the keyserver and examine content on olm_sessions table. New entry should appear.

Reviewers: tomek, atul, bartek, jon, ashoat

Reviewed By: ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D7654