Make it possible to view read receipts

[neiljp]

This issue tracks supporting the read receipts feature which is likely to release in Zulip 6.0.

The equivalent mobile issue is zulip/zulip-mobile#5367
In the server this was covered by zulip/zulip#18935, among others.

This should appear in Feature Level 137 in the API Changelog
(https://chat.zulip.org/api/changelog#changes-in-zulip-60)

As per this API, this is an on-demand feature, so client data won't get updated via events. Due to this is seems likely that this could be accessed from the message information popup, in a popup from there (like edit history).

This is an initial idea however, so please discuss further in #zulip-terminal first on chat.zulip.org.