Only owner can change app pay-account (bug 840188)

This branch changes payment accounts so that only an app owner can change the payment account.

If a payment account is set by an owner and an non-owner looks at the page then the account name is displayed outside of the select - this makes it clearer that the account set is not part of the user's list of accounts.

I've also fixed a small issue where the js to disable the form fields was being overridden by the price selection js.