Make host_meta/webfinger replies cacheable (fixes #6100) (#6101)

* 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

app/controllers/well_known/host_meta_controller.rb

 # frozen_string_literal: true
 
 module WellKnown
-  class HostMetaController < ApplicationController
+  class HostMetaController < ActionController::Base
     include RoutingHelper
 
+    before_action { response.headers['Vary'] = 'Accept' }
+
     def show
       @webfinger_template = "#{webfinger_url}?resource={uri}"
 
       respond_to do |format|
         format.xml { render content_type: 'application/xrd+xml' }
       end
+
+      expires_in(3.days, public: true)
     end
   end
 end

app/controllers/well_known/webfinger_controller.rb

 # frozen_string_literal: true
 
 module WellKnown
-  class WebfingerController < ApplicationController
+  class WebfingerController < ActionController::Base
     include RoutingHelper
 
+    before_action { response.headers['Vary'] = 'Accept' }
+
     def show
       @account = Account.find_local!(username_from_resource)
 
@@ -16,6 +18,8 @@ module WellKnown
           render content_type: 'application/xrd+xml'
         end
       end
+
+      expires_in(3.days, public: true)
     rescue ActiveRecord::RecordNotFound
       head 404
     end