Tweak `isBrowserExtension()` to take just the `config.app` string that
it needs and not the entire `config` object as argument. This is
necessary for `isBrowserExtension()` to be called by `settings.js`
instead of `index.js` (as I want it to be in the future) - when it's
called by `settings.js` the `config` object as a whole won't be
available (and hasn't been constructed yet).