Create a design document for patron block overrides

It's been decided that block overrides during check out will be implemented in two steps.

Step 1 (included in R1): implement item blocks using the existing approach to overrides. This is possible because two of the proposed item-related blocks are mutually exclusive, so we'd never need to override both of them.

Step 2 (pushed to R2): implement patron-related blocks. This will require significant changes in the check out implementation (possibly, getting rid of the separate "check out override" endpoint).

The design doc for Step 2 needs to be prepared during R1.