A hidden preference matching the behavior of "browser.download.manager.alertOnEXEOpen" is kept, but is renamed in order to recover cases where the checkbox was used accidentally.
While there, simplify the `confirmLaunchExecutable` function by converting from promises to async function.
Since Basilisk moved this prompting to toolkit, we also clean up some unused duplicate strings from application/basilisk while we're there.
This resolves#581