* 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

* Remove rabl dependency

* Replicate old Oj configuration

This fixes #2375.

This change includes:

- Improve the spec coverage for incoming request to the webfinger action
- For requests without an accept header (ie, what a browser might look like),
  return a JSON response.
- For requests with an explicit format of xml or json, return that format.
- For requests using an accept header, return that format.

Also adds failing spec showing webfinger does not return xml, which covers the
issue described in: https://github.com/tootsuite/mastodon/issues/1983

* 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