people: Extract emails_to_full_names_string helper.

This code that previously lived in a larger function is needed by
another function in PR #22127.

frontend_tests/node_tests/people.js

@@ -664,6 +664,24 @@ test_people("multi_user_methods", () => {
     assert.equal(people.reply_to_to_user_ids_string("invalid@example.com"), undefined);
 });
 
+test_people("emails_to_full_names_string", () => {
+    people.add_active_user(charles);
+    people.add_active_user(maria);
+    assert.equal(
+        people.emails_to_full_names_string([charles.email, maria.email]),
+        `${charles.full_name}, ${maria.full_name}`,
+    );
+
+    assert.equal(
+        people.emails_to_full_names_string([
+            charles.email,
+            "unknown-email@example.com",
+            maria.email,
+        ]),
+        `${charles.full_name}, unknown-email@example.com, ${maria.full_name}`,
+    );
+});
+
 test_people("concat_huddle", () => {
     /*
         We assume that user_ids passed in

static/js/drafts.js

@@ -324,16 +324,7 @@ export function format_draft(draft) {
         };
     } else {
         const emails = util.extract_pm_recipients(draft.private_message_recipient);
-        const recipients = emails
+        const recipients = people.emails_to_full_names_string(emails);
-            .map((email) => {
-                email = email.trim();
-                const person = people.get_by_email(email);
-                if (person !== undefined) {
-                    return person.full_name;
-                }
-                return email;
-            })
-            .join(", ");
 
         formatted = {
             draft_id: draft.id,

static/js/people.js

@@ -256,6 +256,19 @@ export function reply_to_to_user_ids_string(emails_string) {
     return user_ids.join(",");
 }
 
+export function emails_to_full_names_string(emails) {
+    return emails
+        .map((email) => {
+            email = email.trim();
+            const person = get_by_email(email);
+            if (person !== undefined) {
+                return person.full_name;
+            }
+            return email;
+        })
+        .join(", ");
+}
+
 export function get_user_time_preferences(user_id) {
     const user_timezone = get_by_user_id(user_id).timezone;
     if (user_timezone) {