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

docs/glossary.rst

@@ -6,6 +6,26 @@ Glossary
 .. glossary::
    :sorted:
 
+   import/export key
+     Unique key representing a particular type of import/export job,
+     i.e. the source/target and orientation (import vs. export).
+
+     For instance "Wutta → CSV export" uses the key:
+     ``export.to_csv.from_wutta``
+
+     More than one :term:`import handler` can share a key, e.g. one
+     may subclass another and inherit the key.
+
+     However only one handler is "designated" for a given key; it will
+     be used by default for running those jobs.
+
+     This key is used for lookup in
+     :meth:`~wuttasync.app.WuttaSyncAppProvider.get_import_handler()`.
+
+     See also
+     :meth:`~wuttasync.importing.handlers.ImportHandler.get_key()`
+     method on the import/export handler.
+
    import handler
      This a type of :term:`handler` which is responsible for a
      particular set of data import/export task(s).