fix: accept optional outbox/followers/following and any AP actor type

Person struct now deserializes gracefully when outbox, followers, or
following are absent (Threads omits them for some actors). Accepts
Service/Application/Organization/Group in addition to Person.
manually_approves_followers defaults to false when absent.

src/user.rs

@@ -17,9 +17,9 @@ pub struct LookedUpActor {
     pub avatar_url: Option<Url>,
     pub banner_url: Option<Url>,
     pub ap_url: Url,
-    pub outbox_url: Url,
-    pub followers_url: Url,
-    pub following_url: Url,
+    pub outbox_url: Option<Url>,
+    pub followers_url: Option<Url>,
+    pub following_url: Option<Url>,
     pub also_known_as: Option<String>,
     pub profile_url: Option<Url>,
     pub attachment: Vec<ApProfileField>,