Fix Bug 1102367 - CMD+Click does not open a new tab on dev edition download/firstrun page

This is a common issue in our JS code so I'll send a separate PR to add the generic `track_and_redirect` function, which is currently in the [bug-872740-download-btn-ga](https://github.com/kyoshino/bedrock/compare/bug-872740-download-btn-ga) WIP branch.