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

db/migrate/20190914202517_create_account_migrations.rb

@@ -0,0 +1,12 @@
+class CreateAccountMigrations < ActiveRecord::Migration[5.2]
+  def change
+    create_table :account_migrations do |t|
+      t.belongs_to :account, foreign_key: { on_delete: :cascade }
+      t.string :acct, null: false, default: ''
+      t.bigint :followers_count, null: false, default: 0
+      t.belongs_to :target_account, foreign_key: { to_table: :accounts, on_delete: :nullify }
+
+      t.timestamps
+    end
+  end
+end