[WIP] Replace Foxx authentication with Foxx app-based (modular) authentication

This PR replaces the `activateAuthentication`-related features with a substitute based on the new modular Foxx apps but not yet the apps themselves (I'm not sure how to include them while they are maintained in separate repositories).

It also includes the documentation of the apps and the new authentication implementation.

**This PR is backwards-incompatible for all Foxx apps that use `activateAuthentication`.**

We should consider renaming that -- it doesn't actually provide any authentication features other than session handling (including logout) anymore. We could then also consider adding a backwards-compatible legacy wrapper that emulates the old `activateAuthentication` behaviour using the new Foxx apps.