implementing validate references selector, behavior and remote switch, plus update docs

[tomreitz]

This PR implements the additional configuration discussed in this ticket, which allows enumerating specific reference to include or exclude from reference validation, as well as a switch for turning off remote reference validation.

We can use this in assessment bundles like

...
validate:
  methods:
    - references
    - ...
  references:
    selector:
      - studentAssessments.studentReference
    behavior: exclude

to skip validating studentReferences (which should all be valid, given the work done by the student ID package).

I've tested this locally with a few small datasets, and I also (successfully) ran test_lightbeam.py.