refactor: extract signal_alert into its own module

Move signal_alert out of python/common.py into a dedicated
python/signal_alert.py module and update its importers
(validate_system.py, snapshot_manager.py) to the new path.

Relocate the signal_alert tests from tests/test_common.py into
tests/test_signal_alert.py, repatching python.signal_alert.logger and
python.signal_alert.Apprise to match the new module.

python/tools/snapshot_manager.py

@@ -12,7 +12,8 @@ from re import search
 
 import typer
 
-from python.common import configure_logger, signal_alert, utcnow
+from python.common import configure_logger, utcnow
+from python.signal_alert import signal_alert
 from python.zfs import Dataset, get_datasets
 
 logger = logging.getLogger(__name__)