Enhancement: Auth context in route meta props

The route meta prop has been extended by a new "meta.authContext" property (can be one out of "anonymous", "user", "publicLink" or "hybrid"). With this, app developers can now define anonymous routes, which was hardcoded to a few well known route names before.
Anonymous routes are rendered in the application layout, i.e. with the top bar, as the ownCloud Web Chrome should always be visible to the user (except for a few handpicked exceptions in the web runtime, which are still rendered in the plain layout).

https://github.com/owncloud/web/issues/7234
https://github.com/owncloud/web/issues/7863
https://github.com/owncloud/web/pull/7874
