Donate Form block

Refs #3312

* Adds `dosomething_payment_form` which submits a Payment to Stripe

* Adds a Donate Form block which renders the Payment Form with Donate variables.

* Adds a template file for the Donate Form for Paraneue magic

* Creates a `block` subdirectory in the `templates/system` directory to house said block, and our existing `block.tpl.php` override

* Adds a testing function to make this easier to test.

TODO:
* Create a Stripe customer using the form `first_name` and `last_name` fields.
I'm happy to add that in on the next PR, as I'll need to make more API calls to create a Stripe customer for the given First/Last name fields.