Fixing builds, photos, and the composer

A batch of fixes across the builder, your published apps, and the composer:

• Build summaries: builds that used to end with "we could not prepare a reliable summary" now give you a real wrap-up of what changed.
• Linked photos: images stored in an app you linked to now load the first time anyone opens the app, not just for you.
• Error messages: when something goes wrong, the toast now shows the actual reason instead of a generic message.
• Builder composer: pressing Enter sends your message again after you type an @ that matched no file, and the /link menu two-step app picker is back.
• Home screen: the Get started checklist no longer covers the home customizer, and your draft in the home composer sticks around.