Refactor thread responses into an enum.

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

server/src/nm.rs

@@ -14,8 +14,8 @@ use crate::{
     compute_offset_limit,
     error::ServerError,
     graphql::{
-        Attachment, Body, DispositionType, Email, Header, Html, Message, PlainText, Tag, Thread,
-        ThreadSummary, UnhandledContentType,
+        Attachment, Body, DispositionType, Email, EmailThread, Header, Html, Message, PlainText,
+        Tag, ThreadSummary, UnhandledContentType,
     },
     linkify_html, sanitize_html,
 };
@@ -125,7 +125,7 @@ pub async fn thread(
     nm: &Notmuch,
     thread_id: String,
     debug_content_tree: bool,
-) -> Result<Thread, ServerError> {
+) -> Result<EmailThread, ServerError> {
     // TODO(wathiede): normalize all email addresses through an address book with preferred
     // display names (that default to the most commonly seen name).
     let mut messages = Vec::new();
@@ -246,7 +246,7 @@ pub async fn thread(
         .next()
         .and_then(|m| m.subject.clone())
         .unwrap_or("(NO SUBJECT)".to_string());
-    Ok(Thread {
+    Ok(EmailThread {
         thread_id,
         subject,
         messages,