Enhancement: App provider and archiver on public links

We made the app provider and archiver services available on public links. As a prerequisite for this we needed to make backend capabilities available on public links, which will
be beneficial for all future extension development.

https://github.com/owncloud/web/pull/5924
https://github.com/owncloud/web/issues/5884
https://github.com/owncloud/ocis/issues/2479
https://github.com/owncloud/web/issues/2479
https://github.com/owncloud/web/issues/5901
