Date separators

[gnprice]

When successive messages span a date boundary, we should show a separator indicating the new date, like in Zulip web and zulip-mobile.

This complements the dates in recipient headers (#172) by highlighting when the answer changes. (Plus, when we start properly suppressing repeat recipient headers, it makes it possible to do so spanning dates without becoming misleading.)

[gnprice]

In the implementation we merged for #174, we effectively included date separators by having there be a new recipient header when the date changes (even if the recipient didn't).

It'd still be nice to have a distinct design for purely separating dates, like we do in Zulip web and zulip-mobile.

[gnprice]

Early beta feedback reports that our current stopgap use of additional recipient headers as date separators is "distracting":
https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/Flutter.20feedback/near/1700169

So bumping up to Beta 2.

[gnprice]

@terpimost I'm not finding any examples of date separators when I browse the existing Figma design. So I'll base this initially off of what's in web, but I'd be glad to adjust later if you have other thoughts.