Modify ecobenefits to use currency conversion in eco.py
Change BenefitCurrencyConversion to use eco.py currency conversion

Changes model fields to be in line with underlying eco.py factors.

Pushes currency conversion (but not formatting) logic out of the view and
into the ecobenefits helper function.

Adds migrations to drop all existing models and replace them with the
default values for that instance's location.
Update the version of eco.py used