HTH: Allow packaging of data (like an initial sync) into downloadable json.

This is a step in the direction of USB-based syncing and android installs.  It is needed for the HTH deployment of student-specific installs on USB sticks.

Changes:
* Make a generic function for getting models (not serialized models, but models) based on device counters.
* Use that function to get all models on a zone*

Issues:
* This may or may not work with zone_fallback data.  Haven't tested this, but this isn't specific to this PR--this would be a develop branch issue, if so, and part of the ChainOfTrust code.