Refactor ActivityPub handling to prepare for non-Account actors (#19212)

* Move ActivityPub::FetchRemoteAccountService to ActivityPub::FetchRemoteActorService

ActivityPub::FetchRemoteAccountService is kept as a wrapper for when the actor is
specifically required to be an Account

* Refactor SignatureVerification to allow non-Account actors

* fixup! Move ActivityPub::FetchRemoteAccountService to ActivityPub::FetchRemoteActorService

* Refactor ActivityPub::FetchRemoteKeyService to potentially return non-Account actors

* Refactor inbound ActivityPub payload processing to accept non-Account actors

* Refactor inbound ActivityPub processing to accept activities relayed through non-Account

* Refactor how Account key URIs are built

* Refactor Request and drop unused key_id_format parameter

* Rename ActivityPub::Dereferencer `signature_account` to `signature_actor`

spec/controllers/concerns/signature_verification_spec.rb

@@ -3,6 +3,16 @@
 require 'rails_helper'
 
 describe ApplicationController, type: :controller do
+  class WrappedActor
+    attr_reader :wrapped_account
+
+    def initialize(wrapped_account)
+      @wrapped_account = wrapped_account
+    end
+
+    delegate :uri, :keypair, to: :wrapped_account
+  end
+
   controller do
     include SignatureVerification
 
@@ -73,6 +83,41 @@ describe ApplicationController, type: :controller do
       end
     end
 
+    context 'with a valid actor that is not an Account' do
+      let(:actor) { WrappedActor.new(author) }
+
+      before do
+        get :success
+
+        fake_request = Request.new(:get, request.url)
+        fake_request.on_behalf_of(author)
+
+        request.headers.merge!(fake_request.headers)
+
+        allow(ActivityPub::TagManager.instance).to receive(:uri_to_actor).with(anything) do
+          actor
+        end
+      end
+
+      describe '#signed_request?' do
+        it 'returns true' do
+          expect(controller.signed_request?).to be true
+        end
+      end
+
+      describe '#signed_request_account' do
+        it 'returns nil' do
+          expect(controller.signed_request_account).to be_nil
+        end
+      end
+
+      describe '#signed_request_actor' do
+        it 'returns the expected actor' do
+          expect(controller.signed_request_actor).to eq actor
+        end
+      end
+    end
+
     context 'with request older than a day' do
       before do
         get :success