Add `enforce_terms` to `application_settings`

Add a flag to applications settings to enforce users to accept terms
before using the GitLab instance

db/migrate/20180424090541_add_enforce_terms_to_application_settings.rb

+class AddEnforceTermsToApplicationSettings < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def change
+    add_column :application_settings, :enforce_terms, :boolean, default: false
+  end
+end

db/schema.rb

@@ -198,6 +198,7 @@ ActiveRecord::Schema.define(version: 20180502125859) do
     t.string "encrypted_external_auth_client_key_pass"
     t.string "encrypted_external_auth_client_key_pass_iv"
     t.string "email_additional_text"
+    t.boolean "enforce_terms", default: false
   end
 
   create_table "approvals", force: :cascade do |t|