Fix incorrect data type for `SiteSetting.google_oauth2_prompt`.

db/migrate/20180223041147_fix_google_oauth2_prompt_data_type.rb

@@ -0,0 +1,16 @@
+class FixGoogleOauth2PromptDataType < ActiveRecord::Migration[5.1]
+  def up
+    sql = <<~SQL
+    UPDATE site_settings
+    SET data_type=#{SiteSettings::TypeSupervisor.types[:list]}
+    WHERE data_type=#{SiteSettings::TypeSupervisor.types[:enum]}
+    AND name='google_oauth2_prompt'
+    SQL
+
+    execute sql
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end