Create combined inapp page (bug 1059944)

- Add new page which combines inapp keys and products
- Link out to docs
- Disable inapp products pages with waffle
- Add tests


![screenshot 2014-09-09 14 46 10](https://cloud.githubusercontent.com/assets/119884/4206950/a449f232-3851-11e4-89f9-fce6e19e97b0.png)
![screenshot 2014-09-09 14 46 17](https://cloud.githubusercontent.com/assets/119884/4206953/a6a9d3c6-3851-11e4-8cc0-d58175a69a9f.png)
![screenshot 2014-09-09 14 46 23](https://cloud.githubusercontent.com/assets/119884/4206955/a8d270ae-3851-11e4-9549-6e0ec5e23268.png)


WIth in-app-products waffle disabled:
![screenshot 2014-09-09 13 02 44](https://cloud.githubusercontent.com/assets/119884/4205691/27c58fea-3843-11e4-824b-c0f53314cf34.png)


@cvan @mstriemer @andymckay @kumar303 


we tried saving a redirect some time ago by linking to `"https://developer.mozilla.org/" + request.LANG + "/..."` but MDN's locales are really finicky and will just 404 if a page doesn't exist for a particular locale. like this one:

    https://developer.mozilla.org/zh-TW/Marketplace/Monetization/In-app_payments_section/mozPay_iap

I should file a bug against MDN for that.