Add bitcoin option to General Funding

[jezekee]

Please add a Bitcoin button in the General Funding section on the Get Involved page, perhaps above the Paypal button (depends on which funding option is financially preferable).

If I have the option to pay using BTC when downloading but choose $0, it would be best if this option remains open should I choose to pay/donate later on. The way of doing so by going through the download button seems awkward.

Also, if the stripe code allows it, an option to pay with the email being optional would also be appreciated.

Similar to issue #624.

[danirabbit]

Since Stripe does handle Bitcoin, I'm closing this as a duplicate of #624