msglist: Offer starred-messages narrow

[gnprice]

Like the "Starred messages" narrow the web app offers at the top of the left sidebar.

We should support this narrow in the message list, and then provide a way to navigate to it.

If we do this before building a helpful overall navigation for the app, the way to navigate to it can be just another button on HomePage.

This is a sort of followup to #72.

[PIG208]

Completed via #870.