Show "deleted" users specially

The server now has a notion of a user being "deleted" but still in the database, and still in the list of users sent to clients. This is a step beyond the existing "deactivated" state; the relevant differences are:

• Data like the name and avatar will have been scrubbed, replaced with placeholder data.
• The new is_deleted field on the user object will be true, to signal that this happened.

We should look for this field, and when present avoid consulting other fields like user.fullName. Instead, we'll show e.g. "(deleted user)" for the name (akin to the existing "(unknown user)").

References

• #api documentation > new feature level: 490
• #api design > flag for deleted users in UserProfile
• zulip/zulip#38374