Support new settings for restricting direct messages

[gnprice]

This is an upcoming Zulip feature that's expected to appear in Zulip Server 9.0. We'll want to support it properly in zulip-flutter, and we might as well do so now while it's fresh in mind.

The corresponding zulip/zulip and zulip-mobile issues are:

• Add settings for restricting direct messages zulip#24467
• Support new settings for restricting direct messages zulip-mobile#5870

The UX for this should be similar to #675, which is a different feature that also prevents sending DMs to certain users.

[gnprice]

Pushing this back to the "Launch" milestone (i.e. post-summer), given its dependency on #814.

In the meantime, we'll aim to do #815. That will mitigate this issue in a similar way to how we ended up handling it in zulip-mobile (for zulip/zulip-mobile#5870); and it's something we should have in any case.