Clean up rendering of credit card form

Summary:
General cleanup and separation into generic vs Stripe blocks of code.

• There was an old CC form view for Stripe stuff that I never cleaned up; clean that up.
• Move non-Stripe CC form rendering into a base class (Balanced can reuse it).
• Move non-Stripe CC form JS into a shareable class.
• Simplify JS a bit (JX.Workflow can add extra parameters to a request, so we don't need hidden inputs).
• Genericize CSS.
• Depend on Stripe JS directly, if they're down we're not going to be able to add cards anyway.

Ref T2787.

Test Plan: Hit all Stripe errors and added new cards.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2787

Differential Revision: https://secure.phabricator.com/D5758