Move credit card components out of this repo

I think this repo should just be for presentational components, but because it's trying to handle a credit card UI, we've also added libraries for wp.com API support (which are outdated now), store transactions, etc. I think these would be easier to keep up-to-date in the context of their own projects.

It looks like the only place we're using the credit card component is in Akismet, so my suggestion is to move it and all the extras out of this repo, into that one. This would include:

- `components/payment`
- `lib/credit-card-details`
- `lib/paygate-loader`
- `lib/store-transactions`
- `lib/stored-cards`
- `lib/wp`
- `lib/wpcom-undocumented`

I just wanted to start a discussion about whether this is worthwhile, if other projects are using the credit card functionality... let me know 😄