PayPal sidebar integration

NOTE: Paypal Subscription isn't available in every country for private clients

To setup the Subscription feature:

1. Login to paypal.com

2. Go to Merchant Services and create a new subscription button.

3. Put in a price and a name, leave other fields to default.

None of the other defaults need to be tweaked, so save your button and add the button ID to your diaspora.toml by following the example in diaspora.toml.example