Application Manager: Allow user to customise PassengerStartupFile

As a systems administrator, I would like an option to modify the PassengerStartupFile directive via the WHM interface so that I can choose to allow Passenger to run node apps that use a custom entry point..

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Some nodejs frameworks use an entrypoint other than 'app.js' (e.g. nuxt) which means that they can't easily be deployed and run via Application Manager.

I would like an option to modify the PassengerStartupFile directive via the UI, to allow Passenger to run node apps that use a custom entrypoint.

Currently this can be worked around by adding the directive to:

/etc/apache2/conf.d/userdata/std/2_4/<user>/<domain>/<appname.conf>