Add a first draft of the federation page.

This includes the generation of metadata for all hosted entities, even if there's more than one entity defined for a certain type (e.g. multiple SPs or multiple IdPs).

Still missing:

* Metadata converter moved to the admin module.
* Metadata lookup.
* Showing metadata for trusted entities.
* Downloading certificates.
* Move the "clear my discovery choices" link to the new user interface.