Add table to store olm notifications sessions

Summary: This differential adds new table to MariaDB database on the keyserver to store olm sessions for encrypted notifications.

Test Plan:
To test migrations-config.js restart your keyserver terminal. Ensure DB version: 31 log line is visible.
To test setup-db.js create new database and update secrets folder so that it points to new database instance. Start your keyserver.

In both cases using a database connection tool of choice (I prefer to use mariadb terminal command), ensure olm_notifications_sessions table is present and its structure matches SQL statement used to create it.

Reviewers: kamil, atul, inka, tomek, bartek, ashoat, jon

Reviewed By: atul, ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D7463