Switched stripe handling to use Stripe Elements

[melinath]

For PCI compliance and to avoid having to fill out annoying forms every year, we need to switch to using Stripe Elements. This only affects client-side code.

To test this PR:

• Add payment env vars to your dancerfly activate file from 1password

• Create a demo user (email: demo@dancerfly.com)

• Create a demo event (org slug: demo; event slug: demo-event)

• Create one item for the demo event.

• Connect the demo org to LW stripe account at /demo/edit/payment/

• Verify that setting of publishable key is the same, code-wise.

• Verify that, using test API, you are able to save a credit card or get errors from /billing/

• Verify that, using test API, you are able to check out for an event both with and without saving the card.

[chigby]

When I attempt to pay for an event using a saved card, I get this message:

[chigby]

When I attempt to pay with a non-saved credit card (entering a new one's information on the order summary page), I get this message: