feat: add warnings mode for import/export handlers, commands

can now specify `--warn` for import/export CLI, to get diff email when
changes occur.

this also adds `get_import_handler()` and friends, via app provider.

also declare email settings for the 2 existing importers

tests/importing/test_versions.py

@@ -132,8 +132,8 @@ class TestFromWuttaToVersionBase(VersionTestCase):
         # version object should be embedded in data dict
         data = imp.normalize_target_object(version)
         self.assertIsInstance(data, dict)
-        self.assertIn("_version", data)
-        self.assertIs(data["_version"], version)
+        self.assertIn("_objref", data)
+        self.assertIs(data["_objref"], version)
 
         # but normal object is not embedded
         data = imp.normalize_target_object(user)