Add federation relay support (#7998)

* Add federation relay support

* Add admin UI for managing relays

* Include actor on relay-related activities

* Fix i18n

app/controllers/admin/relays_controller.rb

@@ -0,0 +1,58 @@
+# frozen_string_literal: true
+
+module Admin
+  class RelaysController < BaseController
+    before_action :set_relay, except: [:index, :new, :create]
+
+    def index
+      authorize :relay, :update?
+      @relays = Relay.all
+    end
+
+    def new
+      authorize :relay, :update?
+      @relay = Relay.new(inbox_url: Relay::PRESET_RELAY)
+    end
+
+    def create
+      authorize :relay, :update?
+
+      @relay = Relay.new(resource_params)
+
+      if @relay.save
+        @relay.enable!
+        redirect_to admin_relays_path
+      else
+        render action: :new
+      end
+    end
+
+    def destroy
+      authorize :relay, :update?
+      @relay.destroy
+      redirect_to admin_relays_path
+    end
+
+    def enable
+      authorize :relay, :update?
+      @relay.enable!
+      redirect_to admin_relays_path
+    end
+
+    def disable
+      authorize :relay, :update?
+      @relay.disable!
+      redirect_to admin_relays_path
+    end
+
+    private
+
+    def set_relay
+      @relay = Relay.find(params[:id])
+    end
+
+    def resource_params
+      params.require(:relay).permit(:inbox_url)
+    end
+  end
+end