Add UI for setting up account migration (#5832)

app/controllers/settings/migrations_controller.rb

@@ -0,0 +1,32 @@
+# frozen_string_literal: true
+
+class Settings::MigrationsController < ApplicationController
+  layout 'admin'
+
+  before_action :authenticate_user!
+
+  def show
+    @migration = Form::Migration.new(account: current_account.moved_to_account)
+  end
+
+  def update
+    @migration = Form::Migration.new(resource_params)
+
+    if @migration.valid?
+      if current_account.moved_to_account_id != @migration.account&.id
+        current_account.update!(moved_to_account: @migration.account)
+        ActivityPub::UpdateDistributionWorker.perform_async(@account.id)
+      end
+
+      redirect_to settings_migration_path
+    else
+      render :show
+    end
+  end
+
+  private
+
+  def resource_params
+    params.require(:migration).permit(:acct)
+  end
+end