- Remove groupviews, so that they'll get recreated upon reconnection
- Don't call `clearSession` when reconnecting. We want to reuse the
  Backbone collections.
- Emit `rosterInitialized` event and use that in converse-rosterview.js
  (instead of overriding).
- Refactor `onReconnected` to emit `rosterReadyAfterReconnection`, call
  `converse.populateRoster()` and `converse.chatboxes.onConnected()`.
- Recreate the roster view on the `rosterReadyAfterReconnection` event.