Make default app that gets loaded in web configurable

[pascalwengerter]

Is your feature request related to a problem?

According to owncloud/web#5805 (comment), the order of the apps array in the web config determines which one gets loaded on default (the first one in the array, basically). We usually want it to be the files app by default but admins should be able to configure it to their likings

Describe the solution you'd like

The default app should be configurable via config param / env variable / CLI flag.

Describe alternatives you've considered

Documenting the current behavior very well but that seems less ideal

[kulmann]

We already have the config option in web for defining the default app, see

https://github.com/owncloud/web/blob/808be1ac3bed026d7ca1d2f0820f989d97665e4a/packages/web-runtime/src/container/bootstrap.ts#L207

It doesn't have documentation though.

Might be sufficient for now. We could finetune the identification of a default app by at least skipping app ids that are file editors, i.e. only allowing apps to become the default that can be opened without additional context.

Nonetheless, having an env var for specifying the default app is a good idea in any case.

[pascalwengerter]

@kulmann do you want to move it to owncloud/web#7930?