Add migration to fix species related API errors

In 97b4ea759c some species fields were renamed. These field names were
embedded, as strings, in both FieldPermission models and the Instance
mobile field configuration dictionaries, causing API request to break.

The migration in this commit updates the string values to match the new
Species field names.

Fixes #1769
Merge pull request #1770 from jwalgran/topic/fix-api-error-after-renaming-species-fields

Add migration to fix species related API errors
In importer, separate `species.py` and `trees.py` from models.py
Merge pull request #1774 from RickMohr/split-importer-models

In importer, separate `species.py` and `trees.py` from models.py
Merge remote-tracking branch 'OpenTreeMap/release'