Add ability to close network tabs

Proposal for #613 
This appears to work from my testing, with some minor issues:
* Styling gets weird when all networks are closed. It goes back to normal once you connect again.
* The part (x) span does not seem to render correctly on server tabs.
You actually replace `_kiwi.app.connection.active_connection` with `this.model.network` here as that's the associated network.