Content offsetting and consistency token bub
Give error message when extract fails - less "Infinite loading"
Fix model name reverse schema lookup for Clinical advice.

refs openhealthcare/elcid#224
Try to ensure the target model on an extract query clause is a patient/episode subrecord, not a similarly named lookup list.

refs openhealthcare/elcid#225
Debugging print on error for extract download
Use data structure for old reversion tags pre migration.

refs openhealthcare/elcid#236
More refinements to consistency tokening.