This commit adds the new stripe based pricing table.
If the user is logged in, the email field will be prepopulated with the users email.
The pricing table can be configured in the stripe dashboard.
Once the discourse_subscriptions_pricing_table setting is filled with the pricing table embed code from the stripe dashboard,
the pricing table will be displayed on /subscriptions
For more details see https://stripe.com/docs/payments/checkout/pricing-table
spirobel
Jarek Radosz
Blake Erickson