[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 -- @fceller indicated that git submodules might not work).~~

~~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 (e.g. to something like `enableSessions` -- or expand on the "middleware" idea and have middleware that does more than just handling a request) -- it doesn't actually provide any authentication features other than session handling (including logout) anymore. We could then also consider adding a fancy backwards-compatible legacy wrapper that emulates the old `activateAuthentication` behaviour using the new Foxx apps (or just restore the previous implementation).~~

See below.