Resolve business logic leaks ofweb-app-external

[kulmann]

Describe the technical debt

Looking at the CreateAndUpload.vue component in web-app-files and the useFilesActions.ts composable in web-pkg we have some business logic of web-app-external which leaked to the outside of the app.

Expected behavior

ownCloud Web offers the possibility for web-app-external to register create file and open file actions as an extension. The files app and the useFileActions composable should then utilize those in a generic way instead of referencing e.g. the vuex store of the external app (grep for External/mimeTypes).

Actual behavior

Files app and useFileActions composable depend on the external app.