Fuse recipient header into app bar for topic narrows

[gnprice]

Currently when viewing the message list for a topic narrow, we have an app bar that shows both the channel and the topic, and then a recipient header that shows the topic. (And both app bar and recipient header have the channel's subscription color.)

This looks kind of redundant. Especially so when the thread is long enough to fill the screen, so that the recipient header reaches the top — then the topic name gets displayed twice, one right below the other, at the top of the screen. It'd be good to eliminate the redundancy.

Vlad has made prototype designs for us that do that; see previous issue zulip/zulip-mobile#5511. Here's a screenshot:

Design questions we'll want to resolve before implementing this include:

• Have an example where the thread is shorter than the screen, so there's empty space between the app bar and the first message; make sure we're happy with how that looks, or adjust something. (Possibly switch to pinning the earliest message at the top instead of the latest at the bottom, so the empty space goes between the last message and the compose box.)
• Decide how to handle showing the messages' dates, which currently live in the recipient header.

This is a post-launch issue; it doesn't represent any missing functionality, and I think the redundancy is OK for launch.

[chrisbobbe]

• Have an example where the thread is shorter than the screen, so there's empty space between the app bar and the first message; make sure we're happy with how that looks, or adjust something. (Possibly switch to pinning the earliest message at the top instead of the latest at the bottom, so the empty space goes between the last message and the compose box.)

The message list might also be empty, right, and in that case I think we don't show any recipient headers.