* Make host_meta/webfinger replies cacheable (fixes#6100)
Drop common code for handling users and sessions as webfinger queries
are very basic, public APIs.
Also explicitly mark results as cacheable with “expires_in”.
* Add “Vary: Accept” header for caching since content-negociation is used
* Add request spec for host meta route returning xml
* Add routing spec for xrd routes
* Update well-known routes
* Move webfinger and host-meta actions to their own controllers