Separate mutable writes from lease updates.

Rename "Secure Access Tokens"

Rename "Payment Reference Numbers"

Gonna have to rename the whole project I suppose.

Add a web interface to the storage plugin for initiating PRN redemption

Also more docstrings
Also better test coverage of the non-urlsafe-base64 error case

this, in an attempt to resolve the conflicting versions between requirements.txt and requirements-tests.txt

* flexible schema requirements
* in-memory testing
* improved error checking in the view