Support mark-all-as-read from ZT

[neiljp]

This will be a follow-up to #646 (for the events), after which we can support actions in the UI to trigger these from ZT.

The first step to this will be to add one or more appropriate methods in the model for the actions we're sending to the server, since one can mark different kinds of conversations/narrows as read, all the way up to marking all unread messages one has as read.

Once one or more of those are defined and tested, we can decide how to hook those up to different parts of the UI. We already have a stream popup, and possibly soon a topic popup. For smaller conversations we may consider hotkeys, though a hotkey could arguably too-easily cause loss of unread data.