Add a Crowdfunding Option to Requests

Add a crowdfunding option to requests.

1. Admins can enable crowdfunding for feature requests that have been accepted on an individual basis.

2. Admins set a total cost for implementing the request (like a software feature), and set a minimum amount to contribute.

3. Users can make payments to sponsor the request.

PayPal to start is probably fine, other payment types later. Being able to refund PayPal payments through the UI would be nice. Some kind of user attribution/reputation for sponsoring requests would be good.