FEATURE: introduce dedicated storage and DB constraints for anon users

Previously we used custom fields to denote a user was anonymous, this was
risky in that custom fields are prone to race conditions and are not
properly dedicated, missing constraints and so on.

The new table `anonymous_users` is properly protected. There is only one
possible shadow account per user, which is enforced using a constraint.

Every anonymous user will have a unique row in the new table.

db/migrate/20190529002752_add_unique_constraint_to_shadow_accounts.rb

@@ -0,0 +1,59 @@
+# frozen_string_literal: true
+
+class AddUniqueConstraintToShadowAccounts < ActiveRecord::Migration[5.2]
+
+  def up
+    create_table :anonymous_users do |t|
+      t.integer :user_id, null: false
+      t.integer :master_user_id, null: false
+      t.boolean :active, null: false
+      t.timestamps
+
+      t.index [:user_id], unique: true
+      t.index [:master_user_id], unique: true, where: 'active'
+    end
+
+    rows = DB.exec <<~SQL
+      DELETE FROM user_custom_fields
+      WHERE name = 'shadow_id' AND value in (
+        SELECT value
+        FROM user_custom_fields
+        WHERE name = 'shadow_id'
+        GROUP BY value
+        HAVING COUNT(*) > 1
+      )
+    SQL
+
+    if rows > 0
+      STDERR.puts "Removed #{rows} duplicate shadow users"
+    end
+
+    rows = DB.exec <<~SQL
+      INSERT INTO anonymous_users(user_id, master_user_id, created_at, updated_at, active)
+      SELECT value::int, user_id, created_at, updated_at, 't'
+      FROM user_custom_fields
+      WHERE name = 'shadow_id'
+    SQL
+
+    rows += DB.exec <<~SQL
+      INSERT INTO anonymous_users(user_id, master_user_id, created_at, updated_at, active)
+      SELECT f.user_id, value::int, f.created_at, f.updated_at, 'f'
+      FROM user_custom_fields f
+      LEFT JOIN anonymous_users a on a.user_id = f.user_id
+      WHERE name = 'master_id' AND a.user_id IS NULL
+    SQL
+
+    if rows > 0
+      STDERR.puts "Migrated #{rows} anon users to new structure"
+    end
+
+    DB.exec <<~SQL
+      DELETE FROM user_custom_fields
+      WHERE name in ('shadow_id', 'master_id')
+    SQL
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end