Have data ready on app startup

[gnprice]

This is the successor to the zulip-mobile issue zulip/zulip-mobile#5490. The main goal here is:

• When the user opens the app, we should typically have fresh up-to-date server data to show them.

In zulip-flutter, the current behavior is that we don't store server data at all between runs of the app. Effectively this means it's a lot like the web or desktop app: if you open Zulip in a fresh browser tab, it will similarly take some time (up to like 10 seconds for a large organization like chat.zulip.org, even on a good network) before the app is available.

For discussion, see zulip/zulip-mobile#5490 and this chat thread.

[chrisbobbe]

From beta feedback yesterday:

https://chat.zulip.org/#narrow/channel/48-mobile/topic/cache.20messages.20for.20offline.20use/near/2010675

I wish the app would cache messages more. Right now both this and the old version are nigh unusable offline or in spotty Internet

Greg replied:

I think this is basically another angle on #477, Have data ready on app startup. It's the same feature, but a bit of a different user story for why it's desired: rather than about not waiting for like 5–10 seconds for the data to load, it's about being able to use the app at all (albeit read-only) when not connected.

[PIG208]

This was brought up again in response to the comment here:

One more issue for mobile internet with throatling applied (slow data transfer).
I observerved it takes forever to load Zulip content. At least from time to time. Initial screen offers to use other account. And it helps to use alternative realm hosted on Zulip Cloud.