pm_list: Use "Muted user" placeholder for muted usernames.

This also handles a few other places missed earlier like
narrow headings, beacuse they use the same function.

We already rerender the PM list for events, so there's no
need to do anything special when someone is muted/unmuted.

`people.get_full_names` is now only used in the settings pages
while creating ListWidgets etc, so we add a new test for
it to ensure coverage.

frontend_tests/node_tests/people.js

@@ -397,8 +397,18 @@ test_people("pm_lookup_key", () => {
 
 test_people("get_recipients", () => {
     people.add_active_user(isaac);
+    people.add_active_user(linus);
     assert.equal(people.get_recipients("30"), "Me Myself");
     assert.equal(people.get_recipients("30,32"), "Isaac Newton");
+
+    muting.add_muted_user(304);
+    assert.equal(people.get_recipients("304,32"), "Isaac Newton, translated: Muted user");
+});
+
+test_people("get_full_name", () => {
+    people.add_active_user(isaac);
+    const names = people.get_full_name(isaac.user_id);
+    assert.equal(names, "Isaac Newton");
 });
 
 test_people("get_full_names_for_poll_option", () => {

static/js/people.js

@@ -333,7 +333,7 @@ export function get_recipients(user_ids_string) {
         return my_full_name();
     }
 
-    const names = other_ids.map((user_id) => get_full_name(user_id)).sort();
+    const names = get_display_full_names(other_ids).sort();
     return names.join(", ");
 }