added givie integration behind feature flag

For now, we integrate by just creating a image url with parameters containing donation information. We encrypt the data now with a shared key. This prevents people from tampering.

Not sure if this is the integration we want in the long term for other sites, but it Works For Now™. :)

We deploy this behind a parameter for now (/donations/:id/confirm?givie=true), so we can test with it before going live.

##Deployment instructions
Add the following env variables:
GIVIE_BASE_URL=http://givie.io
GIVIE_SECRET=<make up a sufficiently long secret>

To test this, make a payment and then reload the confirmation page setting the param ?givie=true

To go live we remove the feature flag.