Get valid i-Tree codes by region from ecoservice

Fixes false negatives when an i-Tree code is valid for a region but no OTM code references it.

Requires OpenTreeMap/ecobenefits#43
Fix merging of i-Tree code field

* When storing an i-Tree pair, use `(region, code)` instead of `(code, region)`
* When detecting diffs, use string version of i-Tree code field rather than pair version
* When displaying a diff, for the "imported value" use the value in the diff rather than the value in the row