ui: Add "inbox" view

[chrisbobbe]

Like the "Home" tab / UnreadCards in zulip-mobile.

[chrisbobbe]

As a first pass at least, I plan to implement this design for the screen: https://www.figma.com/file/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=442%3A18163&mode=dev

Unless @terpimost would prefer something different.

[chrisbobbe]

@terpimost, can you point me to a specification for how to compute those stream-specific colors from a base stream color in the user data? Otherwise in this first pass I won't implement stream colors.

[chrisbobbe]

Oh! Never mind; I see this was answered in chat in August: https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/design.3A.20.23F117.20.22Inbox.22.20screen/near/1624484

I've just resumed that conversation with another question I had about the design.