Introduce RecurlyJS module for updating billing information/subscribing

When this module was first written, Recurly only offered the "hosted pages" feature for accepting/updating credit card information without taking on the burden of PCI compliance. Since then, Recurly introduced RecurlyJS (https://js.recurly.com), which provides a mechanism for accepting/updating credit cards without any server-side processing.

As mentioned in #1627922: Add user-facing pages for managing subscriptions and #1621800: Separate out Hosted Pages into Separate Module, the payment system that users choose will vary from site to site. If they want the minimum responsibility, they can use the Managed Pages feature. For better integration directly with the site, users can use this new RecurlyJS module. And for the tightest integration possible (though taking on the greatest level of responsibility), users can use the Commerce Recurly module that @blasthaus has been assembling (not yet publicly released).

This patch gets us started with integrating RecurlyJS. It provides a page for updating billing information, but it does not yet provide a front-facing page for accepting payments. To help bring in more collaboration on the project, we're moving this into the public space so that it can be handled by multiple people.