Add account migration UI (#11846)

Fix #10736

- Change data export to be available for non-functional accounts
- Change non-functional accounts to include redirecting accounts

app/models/account_migration.rb

@@ -0,0 +1,74 @@
+# frozen_string_literal: true
+
+# == Schema Information
+#
+# Table name: account_migrations
+#
+#  id                :bigint(8)        not null, primary key
+#  account_id        :bigint(8)
+#  acct              :string           default(""), not null
+#  followers_count   :bigint(8)        default(0), not null
+#  target_account_id :bigint(8)
+#  created_at        :datetime         not null
+#  updated_at        :datetime         not null
+#
+
+class AccountMigration < ApplicationRecord
+  COOLDOWN_PERIOD = 30.days.freeze
+
+  belongs_to :account
+  belongs_to :target_account, class_name: 'Account'
+
+  before_validation :set_target_account
+  before_validation :set_followers_count
+
+  validates :acct, presence: true, domain: { acct: true }
+  validate :validate_migration_cooldown
+  validate :validate_target_account
+
+  scope :within_cooldown, ->(now = Time.now.utc) { where(arel_table[:created_at].gteq(now - COOLDOWN_PERIOD)) }
+
+  attr_accessor :current_password, :current_username
+
+  def save_with_challenge(current_user)
+    if current_user.encrypted_password.present?
+      errors.add(:current_password, :invalid) unless current_user.valid_password?(current_password)
+    else
+      errors.add(:current_username, :invalid) unless account.username == current_username
+    end
+
+    return false unless errors.empty?
+
+    save
+  end
+
+  def cooldown_at
+    created_at + COOLDOWN_PERIOD
+  end
+
+  private
+
+  def set_target_account
+    self.target_account = ResolveAccountService.new.call(acct)
+  rescue Goldfinger::Error, HTTP::Error, OpenSSL::SSL::SSLError, Mastodon::Error
+    # Validation will take care of it
+  end
+
+  def set_followers_count
+    self.followers_count = account.followers_count
+  end
+
+  def validate_target_account
+    if target_account.nil?
+      errors.add(:acct, I18n.t('migrations.errors.not_found'))
+    else
+      errors.add(:acct, I18n.t('migrations.errors.missing_also_known_as')) unless target_account.also_known_as.include?(ActivityPub::TagManager.instance.uri_for(account))
+      errors.add(:acct, I18n.t('migrations.errors.already_moved')) if account.moved_to_account_id.present? && account.moved_to_account_id == target_account.id
+      errors.add(:acct, I18n.t('migrations.errors.move_to_self')) if account.id == target_account.id
+    end
+  end
+
+  def validate_migration_cooldown
+    errors.add(:base, I18n.t('migrations.errors.on_cooldown')) if account.migrations.within_cooldown.exists?
+  end
+end