Move credits to own list item

After a chat with @jerith (thanks!), moved `100,000,000 credits` to it's own bit.

> Help | Account | 100,000,000 credits

Looks like this: https://www.dropbox.com/s/zg3b2bm2swwoaa9/Screenshot%202014-12-09%2006.32.31.png?dl=0

Also made it a link to `/account/billing/`.