Show read receipts

[gnprice]

This Zulip feature:
https://zulip.com/help/read-receipts

The API endpoint for this feature is:
https://zulip.com/api/get-read-receipts
Unlike most Zulip features, this data isn't controlled by the event system; we just fetch it separately from the server.

See the zulip-mobile implementation for a model to work from, in src/message/ReadReceiptsScreen.js. We can use similar logic: in particular, the data won't live on PerAccountStore at all, but only in the state of the StatefulWidget for the read-receipts screen.

[chrisbobbe]

When this is done we should update the Help Center doc. We removed instructions for the legacy app when preparing to launch the new app: zulip/zulip@a10bcdf

[chrisbobbe]

Someone wrote to support@ in the early launch period (~1 day) requesting this feature. Bumping to M6, since this was supported in the legacy app.