Still lacks inter-tab communication to update state across tabs, i.e.
when sending a 1-on-1 message in one tab, it doesn't appear in another,
because that information is not available via the websocket connection.

- Create a new `Connection` class that extends Strophe.Connection and
    move related code from `converse-core.js` into this class.
- Store the session in localStorage when using a worker
- Move XEP-0156 code to connection.js
    This allows us to initialize the connection without needing to know the
    domain.