Rake task to export class_hash and learner keys for adding to LARA Runs.
Rake task to export class_hash and learner keys for adding to LARA Runs.

[#165217423]

https://www.pivotaltracker.com/story/show/165217423
The idea is to update about ~500K learner-runs in LARA to include
`class_hash` values, which we need for FireStore authorization rules.

This commit is part of that story. Just exporting the values we need to use
later in this format:

 `CLAZZ_ID, CLASS_HASH, LEARNER_ID, LEARNER_KEY`

concord-consortium/rigse,concord-consortium/rigse,concord-consortium/rigse,concord-consortium/rigse,concord-consortium/rigse,concord-consortium/rigse