msglist: Offer stream and topic narrows

[gnprice]

Currently the prototype only offers the message list for one narrow, the "All messages" narrow.

We should offer more narrows. In particular:

• a stream
• a topic in a stream
• a DM conversation (including a group-DM conversation) → msglist: Offer DM-conversation narrows #142

(Also @-mentions, starred messages, and searches, but those can be separate issues later. -> #250, #251, #252)

This is especially needed because it's part of having a reasonable UX for participating in a conversation.

For this to be a really good UX, it'll probably need to be paired with something like an inbox or recent-conversations view. But to keep the scopes disentangled, for the purpose of this issue it'd be OK to skip those. There'll need to be some kind of navigation in order to exercise the feature, but it can be something crude like a few additional buttons on the app's home screen to go alongside the "All messages" button.

Things that are in scope for this issue include:

• fetching the appropriate list of messages for a given narrow
• maintaining that list as message events come in
• adapting the compose box's layout and behavior

[gnprice]

I've just narrowed the scope of this issue somewhat, splitting out the rest to:

• msglist: Offer DM-conversation narrows #142

[gnprice]

Also split out:

• Compose box for topic (and DM) narrows #144