feature: add free charge price mode for product buy page

[hsluoyz]

Right now, users can pay static price at product buy page: https://door.casdoor.com/products/built-in/product_casbin_software/buy

But some other users would like to pre-pay a customized price, it will go into user's balance. And the user can then buy the product by costing the balance.

1. The user should have different balances for currencies like USD, EUR, SGD, etc.
2. The charge page is also a "special" product buy page.

Like:

[casbin-bot]

@seriouszyx @ComradeProgrammer @Resulte