Paid store of free products

It is useful to supplement grants with a store of products where anyone can set his own price like [this page](https://leanpub.com/socialcrypto).

But unlike LeanPUB our prices should always start from zero.

Rationale: People would be more motivated to pay for what they "purchase" (even if offered free option) than for abstract public goods.

An item for sale should be optionally belonging to a grant and its purchase should count towards its matching amount.