people: Deduplicate get_full_names logic.

Share code between `safe_full_names` and `get_display_full_names`
functions, and rename `safe_full_names` to `get_full_names_for_poll`,
because that was the only place where this was used earlier.

This also has the nice side effect of showing "Muted user" instead
of the muted username in poll results.

static/js/poll_widget.js

@@ -19,7 +19,7 @@ export function activate({
         is_my_poll,
         question,
         options,
-        comma_separated_names: people.safe_full_names,
+        comma_separated_names: people.get_full_names_for_poll_option,
         report_error_function: blueslip.warn,
     });