Refactor Backdrop to use the Performance Platform client's admin API

Here's the tl;dr:

- We now use an API user in Signon rather than a token that's just stored in Stagecraft configuration
- Backdrop now uses a new version of the performanceplatform-client that supports an administrative API

I've deleted quite a lot of code. Part of that is the default values for a data set, which are now a few options at the top of the `read/api.py`. I'm not sure if that's the best way to go about it.

The final commit might look big, but there are 4 times the number of deletions than additions.

This will require additional configuration in each environment before it can be deployed.