New strategy for sandbox mode

This is a major PR that completely refactors the approach to crowdtilt api interactions and how 'sandbox mode' is implemented.  Previously, the entire site would either be in 'sandbox' or 'production' mode, and switching between the two was a one-time operation that couldn't be reversed.  Now, each individual campaign can be in either a sandbox or production mode (sandbox by default) to allow the site owner to test his campaigns more thoroughly before activating live payments.

Also put convenient status indicators and navigation links in the admin toolbar that is only visible when an admin is signed in.