Use custom units and digits in field display and editing

* Add helper functions to units.py
* Make sure plot & tree fields have "in request.instance" so they'll use custom units and digits
* Format display values client-side on form save (digits, units)
* Display eco benefits values using configured units and digits.
* ignore .orig files (generated by kdiff3) and extracollect.py
* Fix inline field tag tests
* Fixes #330, #371, #458
Simplify inline field tag tests
Add tests for custom units and digits