Fix merging of i-Tree code field

In the "merge species" UI, for the i-Tree code "imported value", display just region/code pairs which differ from the species, rather than the raw import value (which may have region/code pairs which match the species).

* 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
* Fix `collected_fields` logic in `views._get_row_data`