Refactor thread responses into an enum.

Lays ground work for different types of views, i.e. email, news, docs, etc.

server/src/graphql.rs

@@ -32,8 +32,13 @@ pub struct ThreadSummary {
     pub tags: Vec<String>,
 }
 
+#[derive(Debug, Union)]
+pub enum Thread {
+    Email(EmailThread),
+}
+
 #[derive(Debug, SimpleObject)]
-pub struct Thread {
+pub struct EmailThread {
     pub thread_id: String,
     pub subject: String,
     pub messages: Vec<Message>,
@@ -369,11 +374,13 @@ impl QueryRoot {
             .field("contentTree")
             .exists();
         // TODO: look at thread_id and conditionally load newsreader
-        if newsreader::is_newsreader_thread(&thread_id) {
-            Ok(newsreader::thread(pool, thread_id).await?)
-        } else {
-            Ok(nm::thread(nm, thread_id, debug_content_tree).await?)
-        }
+        Ok(Thread::Email(
+            if newsreader::is_newsreader_thread(&thread_id) {
+                newsreader::thread(pool, thread_id).await?
+            } else {
+                nm::thread(nm, thread_id, debug_content_tree).await?
+            },
+        ))
     }
 }