presence: Frontend implementation of the last_update_id API.

Note: This involves adding presence info of unknown users to the
presence data.
With some small tweaks, we can just add the info to the presence data
structures, just making sure the buddy list correctly skips those
entries and that we redraw the user in the case where the user creation
event arrives after the presence polling loop.

web/src/activity_ui.ts

@@ -78,6 +78,16 @@ export function redraw_user(user_id: number): void {
     });
 }
 
+export function check_should_redraw_new_user(user_id: number): boolean {
+    if (realm.realm_presence_disabled) {
+        return false;
+    }
+
+    const user_is_in_presence_info = presence.presence_info.has(user_id);
+    const user_not_yet_known = people.maybe_get_user_by_id(user_id, true) === undefined;
+    return user_is_in_presence_info && user_not_yet_known;
+}
+
 export function searching(): boolean {
     return user_filter?.searching() ?? false;
 }