Improve UX for manual build triggering

Currently, there is no quick way to trigger an entire group at once. You can't even open each "Play" link in a new tab because the route apparently only works for XHR requests -- opening them in a new tab results in a 404. You currently have to click one, navigate back, click another, navigate back, and so on. Tedious.

Two things would be great:

1. A top-level "Play" button that executes the entire group
2. Clicking the individual "Play" should just trigger the build in the background, stay on the same page and use the already-existing live updating to provide user feedback. If they want to watch the log they can click through the build detail link.

cc @bikebilly