Managing advertiser billing

[davidfischer]

Advertisers should be able to manage their billing and top-up their ad account to keep ads going.

Features include:

• Advertisers should be able to top up the $ on an account and this will credit the account with the relevant amount of clicks/views based on the money and targeting.
• This will involve integration with Stripe but it should support advertisers that pay by check or other means.
• Advertisers should be able to see how much money is left on a particular ad campaign/flight and when it's projected to finish. (Solved by #113)

Future enhancements

These probably won't be part of the initial changes for this issue but could be future enhancements that we add after.

• Emailing advertisers to let them know that their campaign/flight is about to end so they can top-up their account

[davidfischer]

We now store the stripe_id on the advertiser model and we can create invoices through the admin interface but it isn't self-serve.

[davidfischer]

We now have an advertiser billing interface using the Stripe portal. Advertisers can also see invoices attached to their flights.