api: Get and send messages from arbitrary places, not hard-coded

[gnprice]

(Stacked atop #137.)

This PR extends the API bindings for getMessages and sendMessage so that they cover the API more completely. In particular this means:

• getMessages accepts a general narrow, rather than hardcoding the all-messages narrow.
• sendMessage accepts an arbitrary conversation, rather than only stream conversations and hardcoded to #test here (and limited to chat.zulip.org).

This takes us toward:

• msglist: Offer stream and topic narrows #72

Still to do for that issue, and present in a draft branch:

• adding streams data from the server
• adapting the compose box by narrow
• adapting the message-list app bar by narrow
• making recipient headers touchable to navigate to different narrows
• adding PmNarrow as another Narrow subclass

[chrisbobbe]

This LGTM after #137 is in; please see #137 (comment).