Add UI for setting up account migration (#5832)

app/models/form/migration.rb

@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+class Form::Migration
+  include ActiveModel::Validations
+
+  attr_accessor :acct, :account
+
+  validates :acct, presence: true
+
+  def initialize(attrs = {})
+    @account = attrs[:account]
+    @acct    = attrs[:account].acct unless @account.nil?
+    @acct    = attrs[:acct].gsub(/\A@/, '').strip unless attrs[:acct].nil?
+  end
+
+  def valid?
+    return false unless super
+    set_account
+    errors.empty?
+  end
+
+  private
+
+  def set_account
+    self.account = ResolveRemoteAccountService.new.call(acct) if account.nil? && acct.present?
+  end
+end