Show unread count for each account

[gnprice]

In the UI where we list your accounts, we should show your number of unreads in each account.

The eventual design for this is tracked in #1038, for an account-switcher modal. But this could also be implemented before such a modal — the unread counts would fit just as well on the existing choose-account page.

The semantics for how many unreads you have, and in particular for which messages count toward the total (in the presence of muting, etc.), should be the same as for the desktop app and for the favicon in the web app.

Implementation

Getting the data to know how many unreads the user has in each account will involve creating a PerAccountStore proactively for each account. To do this, we'll make a PerAccountStoreWidget for each account in this UI.

When the PerAccountStore for a given account isn't yet ready — for example because we haven't yet been able to reach that server and load data — we'll want to somehow let the user know that the unread count is still loading. We shouldn't just silently show no unreads, the same way we would if we did know the answer and the answer was zero, because that would mislead the user.

This feature won't be suitable for a contributor new to the project.

[gnprice]

This feature is valuable for people who use a large number of different Zulip accounts, and generally keep their unreads clear. For those users, it makes it possible to quickly spot which accounts have new messages to look at, without having to go into each of them one by one to check.

For example we heard this feedback from a user today in the context of a migration from Slack:

I currently have 22 Slack workspaces that I communicate with all the time. Unfortunately, not using my phone to commuinicate with them is not an option as I am away from my pc for extended periods but work still goes on.

[chrisbobbe]

We've had a request for this in early launch feedback: #1644