Fix anonymous access to outbox not being cached by the reverse proxy (#16458)

* Fix anonymous access to outbox not being cached by the reverse proxy

Up until now, anonymous access to outbox was marked as public, but with a
0 duration for caching, which means remote proxies would only serve from cache
when the server was completely overwhelmed.

Changed that cache duration to one minute, so that repeated anonymous access
to one account's outbox can be appropriately cached.

Also added `Signature` to the `Vary` header in case a page is requested, so
that authenticated fetches are never served from cache (which only contains
public toots).

* Remove Vary: Accept header from webfinger controller

Indeed, we have stopped returning xrd, and only ever return jrd, so the
Accept request header does not matter anymore.

* Cache negative webfinger hits for 3 minutes

app/controllers/well_known/webfinger_controller.rb

@@ -4,7 +4,6 @@ module WellKnown
   class WebfingerController < ActionController::Base
     include RoutingHelper
 
-    before_action { response.headers['Vary'] = 'Accept' }
     before_action :set_account
     before_action :check_account_suspension
 
@@ -39,10 +38,12 @@ module WellKnown
     end
 
     def bad_request
+      expires_in(3.minutes, public: true)
       head 400
     end
 
     def not_found
+      expires_in(3.minutes, public: true)
       head 404
     end