Add external process item scoring

This code adds support for building item scorers from external processes.

- Move the mock item scorer up to be a precomputed item scorer.
- Add an external process item scorer builder that runs a program to build an item scorer.
- Add user and item list DAOs for convenience.
- Make the list of test users available in the training phase. This is useful to be able to know what users to score for.

We don't yet have tests for the external process code.